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LT3751 Capacitor Charger Controller with Regulation FEATURES
n n n n n n n n n n
DESCRIPTION
The LT(R)3751 is a high input voltage capable flyback controller designed to rapidly charge a large capacitor to a user-adjustable target voltage set by the transformer turns ratio and three external resistors. An optional feedback pin can be used to provide a low noise high-voltage regulated output. The LT3751 has an integrated rail-to-rail MOSFET gate driver that allows for efficient operation down to 4.75V. A low 106mV differential current sense threshold voltage accurately limits the peak switch current. Added protection is achieved with user-selectable overvoltage and undervoltage lockouts for both VCC and VTRANS. A typical application can charge a 1000F capacitor to 500V in less than one second. The CHARGE pin is used to initiate a new charge cycle and provides ON/OFF control. The DONE pin indicates when the capacitor has reached its programmed value and the part has stopped charging. The FAULT pin indicates when the LT3751 has shut down due to either VCC or VTRANS voltage exceeding the user-programmed supply tolerances.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents including 6518733 and 6636021.
Charges Any Size Capacitor Low Noise Output in Voltage Regulation Mode Stable Operation Under a No-Load Condition Integrated 2A MOSFET Gate Driver with Rail-to-Rail Operation for VCC 8V Selectable 5.6V or 10.5V Internal Gate Drive Voltage Clamp User-Selectable Over/Undervoltage Detect Easily Adjustable Output Voltage Primary or Secondary Side Output Voltage Sense Wide Input VCC Voltage Range (5V to 24V) Available in a 20-Pin QFN 4mm x 5mm
APPLICATIONS
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High Voltage Regulated Supply High Voltage Capacitor Charger Professional Photoflash Systems Emergency Strobe Security/Inventory Control Systems Detonators
TYPICAL APPLICATION
DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY VTRANS 24V T1 1:10 330F 2 40.2k RVTRANS RDCM CHARGE CLAMP RVOUT VCC LT3751 HVGATE LVGATE CSP CSN FB RBG 732
3751 TA01a
D1
Regulation Mode Load Regulation and Efficiency vs Load Current
500V 0 TO 150mA
500
90
10F x2 18.2k 40.2k
* *
+
100F
498 OUTPUT VOLTAGE (V)
84 EFFICIENCY (%)
OFF ON VCC 24V
0.47F
496
78
TO DONE MICRO FAULT 374k UVLO1 VTRANS 475k OVLO1 374k UVLO2 VCC 475k OVLO2 GND
10F
VCC 6m 715k
494
72
492 OUTPUT VOLTAGE EFFICIENCY 490 0 50 100 LOAD CURRENT (mA)
66
10nF
1.74k
60 150
3751 TA01b
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LT3751 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW RVTRANS UVLO1 RDCM 16 RVOUT 15 NC 21 14 RBG 13 HVGATE 12 LVGATE 11 VCC 7 CLAMP 8 FB 9 10 CSN CSP NC
VCC, CHARGE, CLAMP ..............................................24V DONE, FAULT ............................................................24V LVGATE (Note 8) .......................................................24V VCC - LVGATE..............................................................8V HVGATE ................................................................Note 9 RBG, CSP CSN ............................................................2V , FB ...............................................................................5V Current into DONE Pin ...........................................1mA Current into FAULT Pin ...........................................1mA Current into RVTRANS Pin .......................................1mA Current into RVOUT Pin.........................................10mA Current into RDCM Pin ........................................10mA Current into UVLO1 Pin..........................................1mA Current into UVLO2 Pin..........................................1mA Current into OVLO1 Pin..........................................1mA Current into OVLO2 Pin..........................................1mA Maximum Junction Temperature........................... 125C Operating Temperature Range (Note 2).. -40C to 125C Storage Temperature Range................... -65C to 125C
20 19 18 17 OVLO1 1 UVLO2 2 OVLO2 3 FAULT 4 DONE 5 CHARGE 6
UFD PACKAGE 20-PIN (4mm x 5mm) PLASTIC QFN TJMAX = 125C, JA = 43C/W EXPOSED PAD (PIN 21) IS GND, MUST BE TIED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LT3751EUFD#PBF LT3751IUFD#PBF LEAD BASED FINISH LT3751EUFD LT3751IUFD TAPE AND REEL LT3751EUFD#TRPBF LT3751IUFD#TRPBF TAPE AND REEL LT3751EUFD#TR LT3751IUFD#TR PART MARKING* 3751 3751 PART MARKING* 3751 3751 PACKAGE DESCRIPTION 20-Lead (4mm x 5mm) Plastic QFN 20-Lead (4mm x 5mm) Plastic QFN PACKAGE DESCRIPTION 20-Lead (4mm x 5mm) Plastic QFN 20-Lead (4mm x 5mm) Plastic QFN TEMPERATURE RANGE -40C to 125C -40C to 125C TEMPERATURE RANGE -40C to 125C -40C to 125C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
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LT3751 ELECTRICAL CHARACTERISTICS
PARAMETER VCC Voltage RVTRANS Voltage VCC Quiescent Current RVTRANS, RDCM Quiescent Current (Note 3) Not Switching, CHARGE = 5V Not Switching, CHARGE = 0.3V (Note 4) Not Switching, CHARGE = 5V Not Switching, CHARGE = 0.3V (Note 4) Not Switching, CHARGE = 5V Not Switching, CHARGE = 0.3V Measured at 1mA into Pin, CHARGE = 0V Measured at 1mA into Pin, CHARGE = 0V CHARGE = 24V CHARGE = 5V CHARGE = 0V
l l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VCC = CHARGE = 5V, CLAMP = 0V, unless otherwise noted. Individual 25k resistors tied from 5V VTRANS supply to RVTRANS, RVOUT, RDCM, unless otherwise noted. (Note 2)
CONDITIONS
l l
MIN 4.75 4.75
TYP
MAX 24 65
UNITS V V mA A A A A A V V A A A V V s s V mV
5.5 0 35 40 0 47 0 55 60 425 60
8 1 45 1 52 1
RVOUT Quiescent Current
l
42
UVLO1, UVLO2, OVLO1, OVLO2 Clamp Voltage RVTRANS, RVOUT, RDCM Clamp Voltage CHARGE Pin Current
1 1.5 0.3 20
l l
CHARGE Minimum Enable Voltage CHARGE Maximum Disable Voltage Minimum CHARGE Pin Low Time One-Shot Clock Period VOUT Comparator Trip Voltage VOUT Comparator Overdrive Measured RBG at Pin 2s Pulse Width, RVTRANS, RVOUT = 25k RBG = 0.83k Measured as VDRAIN - VTRANS, RDCM = 25k (Note 5) FB Pin = 0V FB Pin = 1.3V Current Sourced from FB Pin, Measured at FB Pin Voltage (Note 6) IVCC 1A
32 0.955
38 0.98 20
44 1.005 40
l
DCM Comparator Trip Voltage Current Limit Comparator Trip Voltage FB Pin Bias Current FB Pin Voltage FB Pin Charge Mode Threshold FB Pin Charge Mode Hysteresis FB Pin Overvoltage Mode Threshold FB Pin Overvoltage Hysteresis DONE Output Signal High DONE Output Signal Low DONE Leakage Current FAULT Output Signal High FAULT Output Signal Low FAULT Leakage Current UVLO1 Pin Current UVLO2 Pin Current OVLO1 Pin Current OVLO2 Pin Current
350
l l l
600 106 11 64 1.22 1.16 55 1.34 60 5 40 5 5 40 5
900 112 15 300 1.25 1.2 1.38
mV mV mV nA V V mV V mV V
100 7 1.19 1.12 1.29
100k to 5V 100k to 5V DONE = 5V 100k to 5V 100k to 5V FAULT = 5V UVLO1 Pin Voltage = 1.24V UVLO2 Pin Voltage = 1.24V OVLO1 Pin Voltage = 1.24V OVLO2 Pin Voltage = 1.24V
l l l l
200 200 200 200 51.5 51.5 51.5 51.5
mV nA V mV nA A A A A
48.5 48.5 48.5 48.5
50 50 50 50
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LT3751 ELECTRICAL CHARACTERISTICS
PARAMETER UVLO1 Threshold UVLO2 Threshold OVLO1 Threshold OVLO2 Threshold Gate Minimum High Time Gate Peak Pull-Up Current Gate Peak Pull-Down Current Gate Rise Time VCC = 5V, LVGATE Active VCC = 12V, LVGATE Inactive VCC = 5V, LVGATE Active VCC = 12V, LVGATE Inactive 10% 90%, CGATE = 3.3nF (Note 8) VCC = 5V, LVGATE Active VCC = 12V, LVGATE Inactive 90% 10%, CGATE = 3.3nF (Note 8) VCC = 5V, LVGATE Active VCC = 12V, LVGATE Inactive (Note 8): VCC = 5V, LVGATE Active VCC = 12V, LVGATE Inactive VCC = 12V, LVGATE Inactive, CLAMP Pin = 5V VCC = 24V, LVGATE Inactive CGATE = 3.3nF 25mV Overdrive Applied to CSP Pin 4.98 10 5 10 CONDITIONS Measured from Pin to GND Measured from Pin to GND Measured from Pin to GND Measured from Pin to GND
l l l l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VCC = CHARGE = 5V, CLAMP = 0V, unless otherwise noted. Individual 25k resistors tied from 5V VTRANS supply to RVTRANS, RVOUT, RDCM, unless otherwise noted. (Note 2)
MIN 1.195 1.195 1.195 1.195 TYP 1.225 1.225 1.225 1.225 0.7 2.0 1.5 1.2 1.5 40 55 30 30 5 10.5 5.6 10.5 180 500 1.6 MAX 1.255 1.255 1.255 1.255 UNITS V V V V s A A A A ns ns ns ns V V V V ns mV V
Gate Fall Time
Gate High Voltage
11.5 6.5 11.5
Gate Turn-Off Propagation Delay Gate Voltage Overshoot CLAMP Pin Threshold
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3751E is guaranteed to meet performance specifications from 0C to 125C junction temperature. Specifications over the -40C to 125C operating junction temperature range are assured by design characterization and correlation with statistical process controls. The LT3751I is guaranteed over the full -40C to 125C operating junction temperature range. Note 3: A 60V internal clamp is connected to RVTRANS, RDCM, RVOUT, UVLO1, UVLO2, OVLO1 and OVLO2. Resistors should be used such that the pin currents do not exceed the Absolute Maximum Ratings. Note 4: Currents will increase as pin voltages are taken higher than the internal clamp voltage.
Note 5: Refer to Block Diagram for VTRANS and VDRAIN definitions. Note 6: Low noise regulation of the output voltage requires a resistive voltage divider from output voltage to FB pin. FB pin should not be grounded in this configuration. Refer to the Typical Application diagram for proper FB pin configuration. Note 7: The feedback pin has built-in hysteresis that defines the boundary between charge-only mode and low noise regulation mode. Note 8: LVGATE should be used in parallel with HVGATE when VCC is less than or equal to 8V (LVGATE active). When not in use, LVGATE should be tied to VCC (LVGATE inactive). Note 9: Do not apply a positive or negative voltage or current source to HVGATE, otherwise permanent damage may occur.
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LT3751 TYPICAL PERFORMANCE CHARACTERISTICS
VCC Pin Current vs Pin Voltage
7 6 PIN CURRENT (mA) 5 4 3 2 1 0 -40C 25C 125C 0 4 12 8 16 PIN VOLTAGE (V) 20 24
3751 G01
VTRANS Supply Current vs Supply Voltage
150 RVTRANS, RVOUT, RDCM = 25k 145 VCC, CHARGE = 5V IVTRANS = IRVTRANS + IRVOUT + IRDCM IVTRANS CURRENT (A) 140 135 130 125 120 115 110 0 10 20 40 30 PIN VOLTAGE (V) 50 -40C 25C 125C 60
3751 G02
CHARGE Pin Current vs Pin Voltage
450 400 350 CURRENT (A) 300 250 200 150 100 50 0 0 4 8 16 12 PIN VOLTAGE (V) -40C 25C 125C 20 24
3751 G03
CHARGE Pin Minimum Enable Voltage vs Temperature
1.3 1.2 CHARGE PIN VOLTAGE (V) CHARGE PIN VOLTAGE (V) 1.1 1.0 0.9 0.8 0.7 0.6 -40 -20 VCC = 5V VCC = 12V VCC = 24V 0 20 40 60 80 TEMPERATURE (C) 100 120
3751 G04
CHARGE Pin Maximum Disable Voltage vs Temperature
1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 -40 -20 VCC = 5V VCC = 12V VCC = 24V PIN LOW VOLTAGE (mV) 400 350
DONE, FAULT Pin Voltage Low vs Temperature
1mA SINK 300 250 200 150 100 100A SINK 50 10A SINK
0
20 40 60 80 TEMPERATURE (C)
100 120
3751 G05
0 -40 -20
0
20 40 60 80 TEMPERATURE (C)
100 120
3751 G06
VOUT Comparator Trip Voltage vs Temperature
30.8 30.4 30.0 29.6 29.2 28.8 VTRANS = 5V VTRANS = 12V VTRANS = 24V 0 UVLO1 PIN VOLTAGE (V) VDRAIN VOLTAGE (V) RVTRANS, RVOUT = 25.5k (RTOL = 1%) 1.236 1.234
UVLO1 Trip Voltage vs Temperature
50.5 50.4 UVLO1 PIN CURRENT (A) 50.3 50.2 50.1 50.0 49.9 49.8
UVLO1 Trip Current vs Temperature
1.232 1.230 1.228 1.226 VCC = 5V VCC = 12V VCC = 24V 0 20 40 60 80 TEMPERATURE (C) 100 120
3751 G08
28.4 -40 -20
VTRANS = 48V VTRANS = 72V 100 120
3751 G07
VCC = 5V VCC = 12V VCC = 24V 0 20 40 60 80 TEMPERATURE (C) 100 120
3751 G09
20 40 60 80 TEMPERATURE (C)
1.224 -40 -20
49.7 -40 -20
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LT3751 TYPICAL PERFORMANCE CHARACTERISTICS
Current Comparator Trip Voltage (Charge Mode) vs Temperature
109.0 VTH = VCSP - VCSN 13.0
Current Comparator Minimum Trip Voltage (Regulation Mode) vs Temperature
VTH = VCSP - VCSN 12.8 FB = 1.3V 12.6 FB PIN VOLTAGE (V) VTH VOLTAGE (mV) 12.4 12.2 12.0 11.8 11.6 11.4 11.2 11.0 -40 -20 0 20 40 60 80 TEMPERATURE (C) VCC = 5V VCC = 12V VCC = 24V 100 120
3751 G11
FB Pin Regulation Mode Threshold vs Temperature
1.168
108.5 VTH VOLTAGE (mV)
1.164
108.0
1.160
107.5 VCC = 5V VCC = 12V VCC = 24V 107.0 -40 -20 0 20 40 60 80 TEMPERATURE (C) 100 120
3751 G10
1.156 VCC = 5V VCC = 12V VCC = 24V 1.152 -40 -20 0 20 40 60 80 TEMPERATURE (C) 100 120
3751 G12
FB Pin Regulation Mode Hysteresis vs Temperature
60 VCC = 5V VCC = 12V VCC = 24V FB PIN VOLTAGE (V) 1.356 1.354 1.352 1.350 1.348 1.346
FB Pin Overvoltage Mode Threshold Voltage vs Temperature
VCC = 5V VCC = 12V VCC = 24V HYSTERESIS (mV) 61.0
FB Pin Overvoltage Mode Hysteresis vs Temperature
58 HYSTERESIS (mV)
60.6
56
60.2
54
59.8
52
59.4
50 -40 -20
0
20 40 60 80 TEMPERATURE (C)
100 120
3751 G13
1.344 -40 -20
0
20 40 60 80 TEMPERATURE (C)
100 120
3751 G14
59.0 -40 -20
VCC = 5V VCC = 12V VCC = 24V 0 20 40 60 80 TEMPERATURE (C) 100 120
3751 G15
CLAMP Pin Threshold vs Temperature
1.9 VCC = 12V VCC = 24V HVGATE PIN VOLTAGE (V) 11.0 10.9 10.8 10.7 10.6 10.5
HVGATE Pin Clamp Voltage vs Temperature
VCC = 24V CLAMP = 0V
1.8 CLAMP PIN VOLTAGE (V)
1.7
1.6
1.5
1.4 -40
0
40 80 TEMPERATURE (C)
120
3751 G16
10.4 -40 -20
0
20 40 60 80 TEMPERATURE (C)
100 120
3751 G17
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LT3751 TYPICAL PERFORMANCE CHARACTERISTICS
HVGATE Pin Clamp Voltage vs Temperature
5.70 VCC = 12V CLAMP = 12V HVGATE PIN VOLTAGE (V) 0.62 DCM TRIP VOLTAGE (V) 5.65 0.64
DCM Trip Voltage (VDRAIN - VTRANS) vs Temperature (RVTRANS = RDCM = 25k)
VTRANS = 5V VTRANS = 12V VTRANS = 24V VTRANS = 48V
0.60
5.60
0.58
5.55
0.56
5.50 -40 -20
0
20 40 60 80 TEMPERATURE (C)
100 120
3751 G18
0.54 -40
0
40 80 TEMPERATURE (C)
120
3751 G19
PIN FUNCTIONS
OVLO1 (Pin 1): VTRANS Overvoltage Lockout Pin. Senses when VTRANS rises above: VOVLO1 = 1.225 + 50A *ROVLO1 and trips the FAULT latch low, disabling switching. After VTRANS drops below VOVLO1, toggling the CHARGE pin reactivates switching. UVLO2 (Pin 2): VCC Undervoltage Lockout Pin. Senses when VCC drops below: VUVLO2 = 1.225 + 50A *RUVLO2 and trips the FAULT latch low, disabling switching. After VCC rises above VUVLO2, toggling the CHARGE pin reactivates switching. OVLO2 (Pin 3): VCC Overvoltage Lockout Pin. Senses when VCC rises above: VOVLO2 = 1.225 + 50A *ROVLO2 and trips the FAULT latch low, disabling switching. After VCC drops below VOVLO2, toggling the CHARGE pin reactivates switching. FAULT (Pin 4): Open Collector Indication Pin. When either VTRANS or VCC exceeds the user-selected voltage range, a transistor turns on. The part will stop switching. This pin needs a proper pull-up resistor or current source. DONE (Pin 5): Open Collector Indication Pin. When the target output voltage (charge mode) is reached, a transistor turns on. This pin needs a proper pull-up resistor or current source. CHARGE (Pin 6): Charge Pin. Initiates a new charge cycle (charge mode) or enables the part (regulation mode) when driven higher than 1.5V. Bring this pin below 0.3V to discontinue charging and put the part into shutdown. Turn-on ramp rates should be between 10ns to 10ms. CHARGE pin should not be directly ramped with VCC or LT3751 may not properly initiate. CLAMP (Pin 7): Internal Clamp Voltage Selection Pin. Tie this pin to VCC to activate the internal 5.6V gate driver clamp. Tie this pin to ground to activate the internal 10.5V gate driver clamp. FB (Pin 8): Feedback Regulation Pin. Use this pin to achieve low noise voltage regulation. FB is internally regulated to 1.22V when a resistive divider is tied from this pin to the output. FB pin should not float. Tie FB pin to either a resistor divider or ground.
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LT3751 PIN FUNCTIONS
CSN (Pin 9): Negative Current Sense Pin. Senses external NMOS source current. Connect to local RSENSE ground connection for proper Kelvin sensing. The current limit is set by 106mV/RSENSE. CSP (Pin 10): Positive Current Sense Pin. Senses NMOS source current. Connect the NMOS source terminal and the current sense resistor to this pin. The current limit is fixed at 106mV/RSENSE in charge mode. The current limit can be reduced to a minimum 11mV/RSENSE in regulation mode. VCC (Pin 11): Input Supply Pin. Must be locally bypassed with high-grade (X5R or better) ceramic capacitor. The minimum operating voltage for VCC is 4.75V. LVGATE (Pin 12): Low Voltage Gate Pin. Connect the NMOS gate terminal to this pin when operating VCC below 8V. The internal gate driver will drive the voltage to the VCC rail. When operating VCC higher than 8V, tie this pin directly to VCC. HVGATE (Pin 13): High Voltage Gate Pin. Connect NMOS gate terminal to this pin for all VCC operating voltages. Internal gate driver will drive the voltage to within VCC - 2V during each switch cycle. RBG (Pin 14): Bias Generation Pin. Generates a bias current set by 0.98V/RBG. Select RBG to achieve desired resistance for RDCM, RVOUT, and RVTRANS. RVOUT (Pin 16): Output Voltage Sense Pin. Develops a current proportional to output capacitor voltage. Connect a resistor between this pin and drain of NMOS such that: RV VOUT = 0.98 * N * OUT - VDIODE RBG when RVOUT is set equal to RVTRANS, otherwise: RVOUT RV VOUT = N * 0.98 * OUT + VTRANS - 1 RBG RVTRANS - VDIODE where VDIODE = forward voltage drop of diode D1 (refer to Block Diagram). RDCM (Pin 17): Discontinuous Mode Sense Pin. Senses when the external NMOS drain is equal to 20A * RDCM + VTRANS and initiates the next charge cycle. Place a resistor equal to 0.45 times the resistor on the RVTRANS pin between this pin and VDRAIN. RVTRANS (Pin 19): Transformer Supply Sense Pin. Connect a resistor between the RVTRANS pin and the VTRANS supply. Refer to Figure 11 for proper sizing of the RVTRANS resistor. The minimum operation voltage for VTRANS is 4.75V. UVLO1 (Pin 20): VTRANS Undervoltage Lockout Pin. Senses when VTRANS drops below: VUVLO1 = 1.225 + 50A *RUVLO1 and trips the FAULT latch low, disabling switching. After VTRANS rises above VUVLO1, toggling the CHARGE pin reactivates switching. GND (Pin 21): Ground. Tie directly to local ground plane.
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LT3751 BLOCK DIAGRAM
47F x2 RVTRANS 40.2k RVTRANS 0.98V REFERENCE 10F SECONDARY PRIMARY VTRANS 12V T1 1:10
D1
+
*
VOUT 450V
+
COUT
OFF ON OTLO VCC 12V 10F 100k VCC DONE SR QQ 100k FAULT MASTER LATCH
START-UP ONE SHOT
DCM COMPARATOR ENABLE GATE DRIVER DCM ONE SHOT
S R FAULT Q Q LATCH
26kHz ONE SHOT CLOCK SQ RQ SWITCH LATCH GATE DRIVE CIRCUITRY
3.8V
+ -
INTERNAL UVLO
VCC RUVLO1 191k VTRANS 55V
UVLO1
+ -
COUNTER
ROVLO1 240k
OVLO1
+
55V
-
RUVLO2 191k VCC 55V UVLO2 UVLO/OVLO COMPARATORS
26kHz ONE SHOT CLOCK
+ -
TO CHARGE 1-SHOT
TIMING AND PEAK CURRENT CONTROL 11mV TO 106mV MODULATION 26kHz ONE SHOT CLOCK ERROR AMP A1
ROVLO2 240k
OVLO2
+
55V
-
1.22V REFERENCE TO VOUT COMPARATOR GND RBG 1.33k RBG DIE TEMP 160C MODE CONTROL
-
+
- +
CHARGE
VOUT COMPARATOR
*
60V RVOUT RVOUT 40.2k
DIFF AMP . COMPARATOR WITH INTERNAL 60V CLAMPS
60V RDCM 60V VDRAIN RDCM 18.2k
1.22V REFERENCE VCC
HVGATE M1
CLAMP VCC LVGATE RESET AUXILIARY CLK COUNT VCC
+
162mV
- +-
MAIN CSP
+
106mV CSN RSENSE 12m
- +-
+ -
1.22V REFERENCE
+
-
FB 10nF
3751 BD
RFBH 3.65M RFBL 10k
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LT3751 OPERATION
The LT3751 can be used as either a fast, efficient high voltage capacitor charger controller or as a high voltage, low noise voltage regulator. The FB pin voltage determines one of the three primary modes: charge mode, low noise regulation, or overvoltage Burst Mode(R) Operation (see Figure 1).
FB PIN VOLTAGE ILPRI IPK VTRANS - VDS(ON) LPRI
ILSEC
OVERVOLTAGE 1.34V REGULATION 1.16V CHARGE MODE 0.0V
3751 F01
IPK N
VOUT + VDIODE LSEC
VPRI
VTRANS - VDS(ON)
Figure 1. FB Pin Modes
CHARGE MODE When the FB pin voltage is below 1.16V, the LT3751 acts as a rapid capacitor charger. The charging operation has four basic states for charge mode steady-state operation (see Figure 2). 1. Start-Up The first switching cycle is initiated approximately 2s after the CHARGE pin is raised high. During this phase, the start-up one-shot enables the master latch turning on the external NMOS and beginning the first switching cycle. After start-up, the master latch will remain in the switching-enable state until the target output voltage is reached or a fault condition occurs. The LT3751 utilizes circuitry to protect against transformer primary current entering a runaway condition and remains in start-up mode until the DCM comparator has enough headroom. Refer to the Start-Up Protection section for more detail. 2. Primary Side Charging When the NMOS switch latch is set, and depending on the use of LVGATE, the gate driver rapidly charges the gate pin to VCC - 2V in high voltage applications or directly to VCC in low voltage applications (refer to the Application
Burst Mode is a registered trademark of Linear Technology Corporation. -N (VTRANS - VDS(ON)) VSEC
-(VOUT + VDIODE) N
VOUT + VDIODE
VDRAIN
V + VDIODE VTRANS + OUT N VTRANS
VDS(ON)
VDS(ON)
3751 F02
1. PRIMARY-SIDE CHARGING
3. 2. DISCONTINUOUS SECONDARY MODE ENERGY TRANSFER DETECTION AND OUTPUT DETECTION
Figure 2. Idealized Charging Waveforms
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LT3751 OPERATION
Information section for proper use of LVGATE). With the gate driver output high, the external NMOS turns on, forcing VTRANS - VDS(ON) across the primary winding. Consequently, current in the primary coil rises linearly at a rate (VTRANS - VDS(ON))/LPRI. The input voltage is mirrored on the secondary winding -N * (VTRANS - VDS(ON)) which reverse-biases the diode and prevents current flow in the secondary winding. Thus, energy is stored in the core of the transformer. 3. Secondary Energy Transfer When current limit is reached, the current limit comparator resets the NMOS switch latch and the device enters the third phase of operation, secondary energy transfer. The energy stored in the transformer core forward-biases the diode and current flows into the output capacitor. During this time, the output voltage (neglecting the diode drop) is reflected back to the primary coil. If the target output voltage is reached, the VOUT comparator resets the master latch and the DONE pin goes low. Otherwise, the device enters the next phase of operation. 4. Discontinuous Mode Detection During secondary energy transfer to the output capacitor, (VOUT + VDIODE)/N will appear across the primary winding. A transformer with no energy cannot support a DC voltage, so the voltage across the primary will decay to zero. In other words, the drain of the NMOS will ring down from VTRANS + (VOUT + VDIODE)/N to VTRANS. When the drain voltage falls to VTRANS + 20A * RDCM, the DCM
V VTH1 VDRAIN VTH2
comparator sets the NMOS switch latch and a new charge cycle begins. Steps 2-4 continue until the target output voltage is reached. Start-Up Protection The LT3751 at start-up, when the output voltage is very low (or shorted), usually does not have enough VDRAIN node voltage to trip the DCM comparator. The part in start-up mode uses the internal 26kHz clock and an auxiliary current comparator. Figure 3 shows a simplified block diagram of the start-up circuitry.
FROM AUXILIARY CURRENT COMPARATOR FROM DCM COMPARATOR FROM CLK INCREMENT COUNTER 1 RESET INCREMENT COUNTER 2 FROM GATE DRIVER ON RESET
3751 F03
Figure 3. Start-Up Protection Circuitry
Toggling the CHARGE pin always generates a start-up one-shot to turn on the external switch, initiating the charging process. After the start-up one-shot, the LT3751 waits for either the DCM comparator to generate a one-shot or the output of the start-up protection circuitry going high, which ever comes first. If the switch drain node, VDRAIN, is below the DCM comparator threshold (see Entering Normal Boundary Mode), the DCM comparator will never fire and the start-up circuitry is dominant.
VOUT
DCM 1-SHOT START-UP (DCM THRESHOLD = VTH1) BOUNDARY-MODE (DCM THRESHOLD = VTH2) BELOW VTH2 (WAIT FOR TIME-OUT)
3751 F04
t
Figure 4. DCM Comparator Thresholds
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-
SWITCH LATCH
11
LT3751 OPERATION
At very low output voltages, the boundary-mode switching cycle period increases significantly such that the energy stored in the transformer core is not depleted before the next clock cycle. In this situation, the clock may initiate another switching cycle before the secondary winding current reaches zero and cause the LT3751 to enter continuous-mode conduction. Normally, this is not a problem; however, if the secondary energy transfer time is much longer than the CLK period, significant primary current overshoot can occur. This is due to the non-zero starting point of the primary current when the switch turns on and the finite speed of the current comparator. The LT3751 startup circuitry adds an auxiliary current comparator with a trip level 50% higher than the nominal trip level. Every time the auxiliary current comparator trips, the required clock count between switching cycles is incremented by one. This allows more time for secondary energy transfer. Counter 1 in Figure 3 is set to its maximum count when the first DCM comparator one-shot is generated. If no DCM one-shot is initiated in normal boundary-mode operation during a maximum count of approximately 500s, the LT3751 re-enters start-up mode and the count is returned to zero. Note that Counter 1 is initialized to zero at start-up. Thus, the output of the startup circuitry will go high after one clock cycle. Counter 2 is reset when the gate driver goes high. This repeats until either the auxiliary current comparator increments the required clock count or until VDRAIN is high enough to sustain normal operation described in steps 2 through 4 in the previous section. Entering Normal Boundary Mode The LT3751 has two DCM comparator thresholds that are dependent on what mode the part is in, either start-up mode or normal boundary-mode, and the state of the mode latch. For boundary-mode switching, the LT3751 requires the DCM sense voltage (VDRAIN) to exceed VTRANS by the DCM comparator threshold, VDRAIN: VDRAIN = (40A + IOFFSET) * RDCM - 40A * RVTRANS where IOFFSET is mode dependent. The DCM one-shot signal is negative edge triggered by the switch node, VDRAIN, and indicates that the energy in the secondary winding has depleted. For this to happen, VDRAIN must exceed VTRANS + VDRAIN prior to its negative edge; otherwise, the DCM comparator will not generate a one-shot to initiate the next switching cycle. The part would remain stuck in this state indefinitely; however, the LT3751 uses the start-up protection circuitry to jumpstart switching if the DCM comparator does not generate a one-shot after a maximum time-out of 500s. Figure 4 shows a typical VDRAIN node waveform with a test circuit voltage clamp applied to the output. VTH1 is the start-up threshold and is set internally by forcing IOFFSET to 40A. Once the first DCM one-shot is initiated, the mode latch is set to boundary-mode. The mode latch then sets the clock count to maximum (500s) and lowers the DCM comparator threshold to VTH2 (IOFFSET = 20A). This provides needed hysteresis between start-up mode and boundary-mode operation. LOW NOISE REGULATION Low noise voltage regulation can be achieved by adding a resistive divider from the output node to the LT3751 FB pin. At start-up (FB pin below 1.16V), the LT3751 enters the charge mode to rapidly charge the output capacitor. Once the FB pin is within the threshold range of 1.16V to 1.34V, the part enters into low noise regulation. The switching methodology in regulation mimics that used in the capacitor charging mode, but with the addition of peak current and duty cycle control techniques. Figure 5 shows the steady state operation for both regulation techniques. Figure 6 shows how both techniques are combined to provide stable, low noise operation over a wide load and supply range. During heavy load conditions, the LT3751 sets the peak primary current to its maximum value, 106mV/RSENSE and sets the maximum duty cycle to approximately 95%. This allows for maximum power delivery. At very light loads, the opposite occurs, and the LT3751 reduces the peak primary current to approximately one tenth its maximum value while modulating the duty cycle below 10%. The LT3751 controls moderate loads with a combination of peak current mode control and duty cycle control.
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LT3751 OPERATION
CHARGE MODE
26kHz ONE-SHOT CLK SWITCH ENABLE MAXIMUM PEAK CURRENT 26kHz ONE-SHOT CLK
LIGHT LOAD OPERATION
... ... NO BLANKING
...
SWITCH ENABLE FORCED BLANKING
DUTY CYCLE CONTROL
DUTY CYCLE CONTROL
...
IPRI
... t
IPRI
... tPER 38s t
HEAVY LOAD OPERATION
26kHz ONE-SHOT CLK SWITCH ENABLE FORCED BLANKING IPRI tPER 38s ... t PEAK CURRENT CONTROL ... ... 26kHz ONE-SHOT CLK 110% VOUT, NOM VOUT 105% VOUT, NOM 1/10TH IPK IPRI
NO LOAD OPERATION
...
...
... t
3751 F05
Figure 5. Modes of Operation (Steady State)
ILIM( IMAX
) DUTY CYCLE(
)
95%
NO LOAD OPERATION
1/10 IMAX
10% 0 LIGHT LOAD MODERATE LOAD HEAVY LOAD CHARGE MODE
3751 F06
LOAD CURRENT
Figure 6. Regulation Technique
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LT3751 OPERATION
Periodic Refresh When the LT3751 enters regulation, the internal circuitry deactivates switching when the internal one-shot clock is high. The clock operates at a 1/20th duty cycle with a minimum blank time of 1.5s. This reset pulse is timed to drastically reduce switching frequency content within the audio spectrum and is active during all loading conditions. Each reset pulse guarantees at least one energy cycle. A minimum load is required to prevent the LT3751 from entering overvoltage Burst Mode Operation. Heavy Load Operation The LT3751 enters peak current mode control at higher output load conditions. The control loop maximizes the number of switch cycles between each reset pulse. Since the control scheme operates in boundary mode, the resonant boundary-mode period changes with varying peak primary current: 1 N Period =IPK * LPRI * + VTRANS VOUT and the power output is proportional to the peak primary current: POUT = 1/ 2 *IPK 1 N + VTRANS VOUT Light Load Operation The LT3751 uses duty cycle control to drastically reduce audible noise in both the transformer (mechanical) and the ceramic capacitors (piezoelectric effects). Internal control circuitry forces a one-shot condition at a periodic rate greater than 20kHz and out of the audio spectrum. The regulation loop then determines the number of pulses that are required to maintain the correct output voltage. Figure 5 shows the use of duty-cycle control. No Load Operation The LT3751 can remain in low noise regulation at very low loading conditions. Below a certain load current threshold (Light Load Operation), the output voltage would continue to increase and a runaway condition could occur. This is due to the periodic one-shot forced by the periodic refresh circuitry. By design, the LT3751 has built-in overvoltage protection associated with the FB pin. When the FB pin voltage exceeds 1.34V (20mV), the LT3751 enters overvoltage Burst Mode Operation. Overvoltage Burst Mode Operation does not reset with the one-shot clock. Instead, the pulse train is completely load-dependent. These bursts are asynchronous and can contain long periods of inactivity. This allows regulation at a no-load condition but with the increase of audible noise and voltage ripple. Note that when operating in overvoltage Burst Mode Operation, the output voltage will increase 10% above the nominal output voltage.
Noise becomes an issue at very low load currents. The LT3751 remedies this problem by setting the lower peak current limit to one tenth the maximum level and begins to employ duty-cycle control.
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LT3751 APPLICATIONS INFORMATION
The LT3751 charger controller can be optimized for either capacitor charging only or low noise regulation applications. Several equations are provided to aid in the design process. Safety Warning Large capacitors charged to high voltage can deliver a lethal amount of energy if handled improperly. It is particularly important to observe appropriate safety measures when designing the LT3751 into applications. First, create a discharge circuit that allows the designer to safely discharge the output capacitor. Second, adequately space high voltage nodes from adjacent traces to satisfy printed circuit board voltage breakdown requirements. Selecting Operating Mode Tie the FB pin to GND to operate the LT3751 as a capacitor charger. In this mode, the LT3751 charges the output at peak primary current in boundary mode operation. This constitutes maximum power delivery and yields the fastest charge times. Power delivery is halted once the output reaches the desired output voltage set by the RVOUT and RBG pins. Tie a resistor divider from the FB pin to VOUT and GND to operate the LT3751 as a low noise voltage regulator (refer to Low Noise regulation section for proper design procedures). The LT3751 operates as a voltage regulator using both peak current and duty cycle modulation to vary output current during different loading conditions. Selecting Component Parameters Most designs start with the initial selection of VTRANS, VOUT, COUT, and either charge time, tCHARGE, (capacitor charger) or POUT,MAX (regulator). These design inputs are then used to select the transformer ratio, N, the peak primary current, IPK, and the primary inductance, LPRI. Figure 7 can be used as a rough guide for maximum power output for a given VTRANS and IPK.
VTRANS (V) 100 90 80 70 60 50 40 30 20 10 0 1 10 PEAK PRIMARY CURRENT (A) 100
3751 F07
P = 20 WATTS P = 50 WATTS P = 100 WATTS
Figure 7. Maximum Power Output
Selecting Transformer Turns Ratio The transformer ratio, N, should be selected based on the input and output voltages. Smaller N values equate to faster charge times and larger available output power. Note that drastically reducing N below the VOUT/VTRANS ratio will increase the flyback voltage on the drain of the NMOS and increase the current through the output diode. A good choice is to select N equal to VOUT/VTRANS. V N OUT VTRANS Choosing Capacitor Charger IPK When operating the LT3751 as capacitor charger, choose IPK based on the required capacitor charge time, tCHARGE, and the initial design inputs. IPK =
(2 * N * VTRANS + VOUT ) * COUT * VOUT Efficiency * VTRANS * ( tCHARGE - t d )
The converter efficiency varies over the output voltage range. The IPK equation is based on the average efficiency over the entire charging period. Several factors can cause the charge time to increase. Efficiency is the most dominant factor and is mainly affected by the transformer winding resistance, core losses, leakage inductance, and transistor RDS. Most applications have overall efficiencies above 70%.
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LT3751 APPLICATIONS INFORMATION
The total propagation delay, td, is the second most dominant factor that affects efficiency and is the summation of gate driver on-off propagation delays and the discharge time associated with the secondary winding capacitance. There are two effective methods to reduce the total propagation delay. First, reduce the total capacitance on the secondary winding, most notably the diode capacitance. Second, reduce the total required NMOS gate charge. Figure 8 shows the effect of large secondary capacitance. The energy stored in the secondary winding capacitance is 1/2 * CSEC * VOUT 2. This energy is reflected to the primary when the diode stops forward conduction. If the reflected capacitance is greater than the total NMOS drain capacitance, the drain of the NMOS power switch goes negative and its intrinsic body diode conducts. It takes some time for this energy to be dissipated and thus adds to the total propagation delay. Choosing Regulator Maximum IPK The IPK parameter in regulation mode is calculated based on the desired maximum output power instead of charge time like that in a capacitor charger application. IPK = 2 * POUT(AVG) 1 * Efficiency V + N VOUT
VDRAIN
ISEC NO SEC. CAPACITANCE IPRI SEC. DISCHARGE
3751 F08
t
Figure 8. Effect of Secondary Winding Capacitance
The previous equation guarantees that the VOUT comparator has enough time to sense the flyback waveform and trip the DONE pin latch. Operating VOUT significantly higher then that used to calculate LPRI could result in a runaway condition and overcharge the output capacitor. The LPRI equation is adequate for most regulator applications. Note that if both IPK and N are increased significantly for a given VTRANS and VOUT, the maximum IPK will not be reached within the refresh clock period. This will result in a lower than expect maximum output power. To prevent this from occurring, maintain the condition in the following equation. LPRI < 38s 1 N IPK * + VTRANS VOUT
TRANS
Note that the LT3751 regulation scheme varies the peak current based on the output load current. The maximum IPK is only reached during charge mode or during heavy load conditions where output power is maximized. Transformer Design The transformer's primary inductance, LPRI, is determined by the desired VOUT and previously calculated N and IPK parameters. Use the following equation to select LPRI: LPRI = 3s * VOUT IPK *N
The upper constraint on LPRI can be reduced by increasing VTRANS and starting the design process over. The best regulation occurs when operating the boundary-mode frequency above 100kHz (refer to Operation section for boundary-mode definition).
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LT3751 APPLICATIONS INFORMATION
Table 1. Recommended Transformers
MANUFACTURER Coilcraft www.coilcraft.com PART NUMBER DA2033-AL DA2034-AL GA3459-BL GA3460-BL 750032051 750032052 750310349 750310355 C8117 C8119 PS07-299 PS07-300 DCT15EFD-U44S003 DCT20EFD-U32S003 DCT25EFD-U27S005 SIZE L x W x H (mm) 17.4 x 24.1 x 10.2 20.6 x 30 x 11.3 32.65 x 26.75 x 14 32.65 x 26.75 x 14 28.7 x 22 x 11.4 28.7 x 22 x 11.4 36.5 x 42 x 23 36.5 x 42 x 23 23 x 18.6 x 10.8 32.2 x 27 x 14 32.5 x 26.5 x 13.5 32.5 x 26.5 x 13.5 22.5 x 16.5 x 8.5 30 x 22 x 12 27.5 x 33 x 15.5 MAXIMUM IPRI (A) 5 10 20 50 5 10 20 50 5 10 20 50 5 10 20 LPRI (H) 10 10 5 2.5 10 10 5 2.5 10 10 5 2.5 10 10 5 TURNS RATIO (PRI:SEC) 1:10 1:10 1:10 1:10 1:10 1:10 1:10 1:10 1:10 1:10 1:10 1:10 1:10 1:10 1:10
Midcom www.midcom.com
Sumida www.sumida.com
TDK www.tdk.com
10.000
fMAX = 50kHz fMAX = 100kHz fMAX = 200kHz
LPRI/WATT (H/WATT)
1.000
0.100
Figure 9 defines the maximum boundary-mode switching frequency when operating at a desired output power level and is normalized to LPRI/POUT (H/Watt). The relationship of output power, boundary-mode frequency, IPK, and primary inductance can be used as a guide throughout the design process. RVTRANS & RDCM Selection RVTRANS sets the common-mode reference voltage for both the DCM comparator and VOUT comparator. Select RVTRANS from the design guide in Figure 10. The RVTRANS pin is connected to an internal 40A current source. Pin current increases as the pin voltage is taken higher than the internal 60V zener clamp. Choose 25k for VTRANS voltage below 12V. Choose 40k for VTRANS voltage between 12V and 60V. For VTRANS voltage greater than 60V, choose RVTRANS to limit RVTRANS pin voltage to less than 60V (i.e. 55V). RVTRANS = VTRANS - 55V 40A
0.010
0.001
1
10 PEAK PRIMARY CURRENT (A)
100
3751 F09
Figure 9. Maximum Switching Frequency
150k 125k RVTRANS () 100k 75k 50k 25k SUGGESTED RVTRANS RESISTANCE
RDCM needs to be properly designed in relation to RVTRANS. Improper selection of RDCM can lead to undesired switching operation at low output voltages.
10 20 30 40 50 60 70 80
3751 F10
VTRANS (V)
RDCM = 0.45 * RVTRANS
Figure 10. Suggested RVTRANS Resistance
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LT3751 APPLICATIONS INFORMATION
RVOUT & RBG Selection RVOUT should be selected based on VOUT comparator trip current. RVOUT current should be designed between 100A and 4mA to maintain accuracy. VOUT,TRIP + VDIODE + VTRANS - 55V N 400A creases with output voltage, the average current though the NMOS is greatest when the output is nearly charged and is given by: IPK * VOUT(PK) IAVG,M = 2(VOUT(PK) + N * VTRANS ) See Table 2 for recommended external NMOS transistors. Gate Driver Operation The LT3751 gate driver has an internal, selectable 10.5V or 5.6V clamp with up to 2A current capability (using LVGATE). For 10.5V operation, tie CLAMP pin to ground, and for 5.6V operation, tie the CLAMP pin to the VCC pin. Choose a clamp voltage that does not exceed the NMOS manufacturer's maximum VGS ratings. The 5.6V clamp can also be used to reduce LT3751 power dissipation and increase efficiency when using logic-level FETs. The typical gate driver overshoot voltage is 0.5V above the clamp voltage. The LT3751's gate driver also incorporates a PMOS pull-up device via the LVGATE pin. The PMOS pull-up driver should only be used for VCC applications of 8V or below. Operating LVGATE with VCC above 8V will cause permanent damage to the part. LVGATE is active when tied to HVGATE and allows rail-to-rail gate driver operation. This is especially useful for low VCC applications, allowing better NMOS drive capability. It also provides the fastest rise times, given the larger 2A current capability verses 1.5A when using only HVGATE.
RVOUT =
RBG sets the trip current and is directly related to the selection of RVOUT. RBG = 0.98 * RVOUT RVOUT VOUT,TRIP + VDIODE - 1 - VTRANS RV N TRANS
NMOS Switch Selection Choose an external NMOS power switch with minimal gate charge and on-resistance that satisfies current limit and voltage break-down requirements. The gate is nominally driven to VCC - 2V during each charge cycle. Ensure that this does not exceed the maximum gate to source voltage rating of the NMOS but enhances the channel enough to minimize the on-resistance. Similarly, the maximum drain-source voltage rating of the NMOS must exceed VTRANS + VOUT/N or the magnitude of the leakage inductance spike, whichever is greater. The maximum instantaneous drain current rating must exceed selected current limit. Because the switching period deTable 2. Recommended NMOS Transistors
MANUFACTURER Fairchild Semiconductor www.fairchildsemi.com On Semiconductor www.onsemi.com PART NUMBER FDS2582 FQB19N20L FQP34N20L MTD6N15T4G NTD12N10T4G NTB30N20T4G NTB52N10T4G Si7820DN Si7818DN SUP33N20-60P ID (A) 4.1 21 31 6 12 30 52 2.6 3.4 33
VDS(MAX) (V) 150 200 200 150 100 200 100 200 150 200
RDS(ON) (m) 66 140 75 300 165 81 30 240 135 60
QG(TOT) (nC) 11 27 55 15 14 75 72 12.1 20 53
PACKAGE SO-8 TO-252 TO-220 DPAK DPAK D2PAK D2PAK 1212-8 1212-8 TO-252
Vishay www.vishay.com
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LT3751 APPLICATIONS INFORMATION
Table 3. Recommended Output Diodes
MANUFACTURER Central Semiconductor www.centralsemi.com Fairchild Semiconductor www.fairchildsemi.com On Semiconductor www.onsemi.com Vishay www.vishay.com PART NUMBER CMR1U-10M ES3J ES1G ES1J MURS360 MURA260 MURA160 USB260 US1G US1M GURB5H60 IF(AV) (A) 1 3 1 1 3 2 1 2 1 1 5 VRRM (V) 1000 600 400 600 600 600 600 600 400 1000 600 TRR (ns) 100 35 35 35 75 75 75 30 50 75 30 PACKAGE SMA SMC SMA SMA SMC SMA SMA SMB SMA SMA TO-263AB
Output Diode Selection The output diode(s) are selected based on the maximum repetitive reverse voltage (VRRM) and the average forward current (IF(AV)). The output diode's VRRM should exceed VOUT + N * VTRANS. The output diode's IF(AV) should exceed IPK /2N, the average short-circuit current. The average diode current is also a function of the output voltage. IAVG = IPK * VTRANS 2 * (VOUT + N * VTRANS )
Setting Current Limit Placing a sense resistor from the positive sense pin, CSP , to the negative sense pin, CSN, sets the maximum peak switch current. The maximum current limit is nominally 106mV/RSENSE. The power rating of the current sense resistor must exceed: PRSENSE I2PK * RSENSE 3 VOUT(PK) V OUT(PK) + N * VTRANS
The highest average diode current occurs at low output voltages and decreases as the output voltage increases. Reverse recovery time, reverse bias leakage and junction capacitance should also be considered. All affect the overall charging efficiency. Excessive diode reverse recovery times can cause appreciable discharging of the output capacitor, thereby increasing charge time. Choose a diode with a reverse recovery time of less than 100ns. Diode leakage current under high reverse bias bleeds the output capacitor of charge and increases charge time. Choose a diode that has minimal reverse bias leakage current. Diode junction capacitance is reflected back to the primary, and energy is lost during the NMOS intrinsic diode conduction. Choose a diode with minimal junction capacitance. Table 3 recommends several output diodes for various output voltages that have adequate reverse recovery times.
Additionally, there is approximately a 100ns propagation delay from the time that peak current limit is detected to when the gate transitions to the low state. This delay increases the peak current limit by (VTRANS)(100ns)/LPRI. Sense resistor inductance (LRSENSE) is another source of current limit error. LRSENSE creates an input offset voltage (VOS) to the current comparator and causes the current comparator to trip early. VOS can be calculated as: L VOS = VTRANS * RSENSE LPRIMARY The change in current limit becomes VOS /RSENSE. The error is more significant for applications using large di/dt ratios in the transformer primary. It is recommended to use very low inductance (< 2nH) sense resistors. Several resistors can be placed in parallel to help reduce the inductance.
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LT3751 APPLICATIONS INFORMATION
Care should also be taken in placement of the sense lines. The negative return line, CSN, must be a dedicated trace to the low side resistor terminal. Haphazardly routing the CSN connection to the ground plane can cause inaccurate current limit. Under/Overvoltage Lockout The LT3751 provides user-programmable under and overvoltage lockouts for both VCC and VTRANS. Use the equations in the Pin Functions section for proper selection of resistor values. When under/overvoltage lockout comparators are tripped, the master latch is disabled, power delivery is halted, and the FAULT pin goes low. Adequate supply bulk capacitors should be used to reduce power supply voltage ripple that could cause false tripping during normal switching operation. The LT3751 provides internal zener clamping diodes to protect itself in shutdown when VTRANS is operated above 55V. Supply voltages should only be applied to UVLO1, UVLO2, OVLO1 and OVLO2 with series resistance such that the Absolute Maximum pin currents are not exceeded. Pin current can be calculated using: IPIN = VAPPLIED - 55V RSERIES NMOS Snubber Design The transformer leakage inductance causes a parasitic voltage spike on the drain of the power NMOS switch during the turn-off transition. Transformer leakage inductance effects become more apparent at high peak primary currents. The worst-case magnitude of the voltage spike is determined by the energy stored in the leakage inductance and the total capacitance on the VDRAIN node. VD,LEAK = LLEAK *I2PK C VDRAIN
Two problems can arise from large VD,LEAK. First, the magnitude of the spike may require an NMOS with an unnecessarily high V(BR)DSS which equates to a larger RDS(ON). Secondly, the VDRAIN node will ring--possibly below ground--causing false tripping of the DCM comparator or damage to the NMOS switch (see Figure 12). Both issues can be remedied using a snubber. If leakage inductance causes issues, it is recommended to use a RC snubber in parallel with the primary winding, as shown in Figure 11. Size CSNUB and RSNUB based on the desired leakage spike voltage, known leakage inductance, and an RC time constant less than 1s. Otherwise, the leakage voltage spike can cause false tripping of the VOUT comparator and stop charging prematurely.
Note that in shutdown, RVTRANS, RVOUT, RDCM, UVLO1, UVLO2, OVLO1 and OVLO2 currents increase significantly when operating VTRANS above the zener clamp voltages and are inversely proportional to the external series pin resistances.
RSNUB LPRI
* *
CSNUB LLEAK
CVDRAIN
3751 F11
Figure 11. RC Snubber Circuit
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LT3751 APPLICATIONS INFORMATION
VDRAIN (WITHOUT SNUBBER) 0V VDRAIN (WITH SNUBBER) 0V IPRI NMOS DIODE CONDUCTS
RFBH, depending on output voltage and type used, may require several smaller values placed in series. This will reduce the risk of arcing and damage to the feedback resistors. Consult the manufacturer's rated voltage specification for safe operation of the feedback resistors. The LT3751 has a minimum periodic refresh frequency limit of 23kHz. This drastically reduces switching frequency components in the audio spectrum. The LT3751 can operate with no-load, but the regulation scheme switches to overvoltage Burst Mode Operation and audible noise and output voltage ripple increase. This can be avoided by operating with a minimum load current. Minimum Load Current Periodic refresh circuitry requires an average minimum load current to avoid entering overvoltage burst mode. Usually, the feedback resistors should be adequate to provide this minimum load current. LPRI *I 2PK * 23kHz ILOAD(MIN) 100 * VOUT IPK is the peak primary current at maximum power delivery. The LT3751 will enter overvoltage burst mode if the minimum load current is not met. Overvoltage burst mode will prevent the application from entering a runaway condition; however, the output voltage will increase 10% over the nominal regulated voltage.
3751 F12
Figure 12. Effects of RC Snubber
Figure 12 shows the effect of the RC snubber resulting in a lower voltage spike and faster settling time. LOW NOISE REGULATION The LT3751 has the option to provide low noise regulated output voltage when using a resistive voltage divider from the output node to the FB pin. Refer to the Selecting Component Parameters section to design the transformer, NMOS power switch, output diode, and sense resistor. Use the following equations to select the feedback resistor values based on the power dissipation and desired output voltage:
RFBH =
( VOUT - 1.22)2
PD
; Top Feedback Resistor
1.22 RFBL = * RFBH ; Bottom Feedback Resistor VOUT - 1.22
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LT3751 APPLICATIONS INFORMATION
Large Signal Stability Large signal stability can be an issue when audible noise is a concern. Figure 13 shows that the problem originates from the one-shot clock and the output voltage ripple. The load must be constrained such that the output voltage ripple does not exceed the regulation range of the error amplifier within one clock period (approximately 6mV referred to the FB pin). The output capacitance should be increased if oscillations occur or audible noise is present. Use Figure 14 to determine the maximum load for a given output capacitance to maintain low audible noise operation. A small capacitor can also be added from the FB pin to ground to lower the ripple injected into FB pin. Small Signal Stability The LT3751's error amplifier is internally compensated to increase its operating range but requires the converter's output node to be the dominant pole. Small signal stability constraints become more prevalent during high-loading conditions where the dominant output pole moves to higher frequency and closer to the internal feedback poles and zeros. The feedback loop requires the output pole frequency to remain below 200Hz to guarantee small signal stability. This allows smaller RLOAD values then the large signal constraint. Thus, small signal issues should not arise if the large signal constraint is met.
30 25 20 15 10 5 26kHz ONE-SHOT CLK
3751 F13
LOAD DROOP VOUT COUT, MIN (F)
VOUT = 150V VOUT = 300V VOUT = 600V
IPRI
0
0
150 50 100 OUTPUT POWER (W)
200
3751 F14
Figure 13. Voltage Ripple Stability Constraint
Figure 14. COUT(MIN) vs Output Power
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LT3751 APPLICATIONS INFORMATION
Board Layout The high voltage operation of the LT3751 demands careful attention to the board layout, observing the following points: 1. Minimize the area of the high voltage end of the secondary winding. 2. Provide sufficient spacing for all high voltage nodes (NMOS drain, VOUT and secondary winding of the transformer) in order to meet the breakdown voltage requirements. 3. Keep the electrical path formed by CVTRANS, the primary of T1, and the drain of the NMOS as short as possible. Increasing the length of this path effectively increases the leakage inductance of T1, potentially resulting in an overvoltage condition on the drain of the NMOS. 4. Reduce the total node capacitance on the RVOUT and RDCM pins by removing any ground or power planes underneath the RDCM and RVOUT pads and traces. Parasitic capacitance can cause unwanted behavior on these pins. 5. Thermal vias should be added underneath the Exposed Pad, Pin 21, to enhance the LT3751's thermal performance. These vias should go directly to a large area of ground plane. 6. Isolated applications require galvanic separation of the output-side ground and primary-side ground. Adequate spacing between both ground planes is needed to meet voltage safety requirements.
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LT3751
RUVLO1 RDCM T1 1:N
VTRANS
ROVLO1 20 RVOUT 16 15 14 LT3751 13 12 11 ANALOG GND M1 7 RSENSE 8 9 10 CVCC VCC RBG ANALOG GND 1 19 18 17
APPLICATIONS INFORMATION
RUVLO2
3 4 5 6 ANALOG GND VIAS
VCC
RFAULT
RDONE
SECONDARY
2
PRIMARY
ROVLO2
CHARGE
CFB RFBL
ANALOG GND
SINGLE POINT GND VOUT
POWER Figure 16. Recommended Board Layout (Not to Scale) R RFBH3 GND RETURN
FBH2
RFBH1
+
+
+
24
CVTRANS2 CVTRANS3 CVTRANS4 POWER GND RETURN RVTRANS REMOVE COPPER FROM ALL SUB-LAYERS (SEE ITEM 4)
POWER GND
CVTRANS1
*
*
POWER GND
DVOUT
CVOUT1
CVOUT2
3751 F15
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LT3751 TYPICAL APPLICATIONS
42A Capacitor Charger
DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY VTRANS 12V TO 24V T1** 1:10 C3 1000F R6 40.2k RVTRANS CHARGE RDCM CLAMP VCC C1 10F R11, 100k DONE R12, 100k R1, 191k VTRANS UVLO1 R2, 475k OVLO1 R3, 191k UVLO2 VCC R4, 475k OVLO2 GND RBG R9 787
3751 TA02
D1
D2***
C2 10F
VOUT 500V C4 1200F
* *
*M1, M2 REQUIRES PROPER HEATSINK/THERMAL DISSIPATION TO MEET MANUFACTURER'S SPECIFICATIONS **THERMAL DISSIPATION OF T1 WILL LIMIT THE CHARGE/DISCHARGE DUTY CYCLE OF C4 ***D2 MAY BE OMITTED FOR OUTPUT VOLTAGE OPERATION BELOW 300V
+
OFF ON VCC 12V TO 24V
R7, 18.2k
LT3751 RVOUT HVGATE LVGATE CSP CSN FB
R8, 40.2k
FAULT
M1, M2* VCC R5 2.5m C1: 25V X5R OR X7R CERAMIC CAPACITOR C2: 25V X5R OR X7R CERAMIC CAPACITOR C3: 25V ELECTROLYTIC C4: HITACHI FX22L122Y 1200F 550V ELECTROLYTIC , D1, D2: VISHAY GURB5H60 600V, 5A ULTRAFAST RECTIFIER M1, M2: 2 PARALLEL VISHAY SUD33N20-60P 200V, 33A NMOS R1 THRU R4: USE 1% RESISTORS R6 THRU R8: USE 1% 0805 RESISTORS R5: USE 2 PARALLEL 5m IRC LR SERIES 2512 RESISTORS T1: COILCRAFT GA-3460-BL 50A SURACE MOUNT TRANSFORMER FOR ANY VOUT VOLTAGE BETWEEN 50V AND 500V SELECT R9 ACCORDING TO: R9 = 0.98 * N * 40.2k VOUT + VDIODE
Charging Efficiency vs Output Voltage
85 1200
Output Capacitor Charge Times vs Capacitance
VOUT = 500V, VTRANS = 24V VOUT = 500V, VTRANS = 12V VOUT = 300V, VTRANS = 24V VOUT = 300V, VTRANS = 12V VOUT = 100V, VTRANS = 24V VOUT = 100V, VTRANS = 12V VOUT 100V/DIV 400 AVERAGE INPUT CURRENT 5A/DIV 0 200 400 600 800 1000 OUTPUT CAPACITANCE (F) 1200
3751 TA02c
Charging Waveform
VOUT = 500V VTRANS = 24V C4 = 1200F
80 CHARGE TIME (ms) VTRANS = 12V VTRANS = 24V 65 50 150 250 350 OUTPUT VOLTAGE (V) 450
3751 TA02b
EFFICIENCY (%)
800
75
70
100ms/DIV
3751 TA02d
3751f
25
LT3751 TYPICAL APPLICATIONS
18A Non-Isolated, High-Voltage Regulator
DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY T1 1:10 C3 680F R6 40.2k C2 5x 2.2F D1 C5 0.47F C4*** 100F VOUT 50V TO 500V
VTRANS 5V TO 24V
* *
+
OFF ON VCC 5V TO 24V
RVTRANS RDCM CHARGE CLAMP LT3751 RVOUT VCC DONE FAULT UVLO1 HVGATE LVGATE CSP CSN FB OVLO2 GND RBG
3751 TA04
R7, 18.2k
*M1 REQUIRES PROPER HEATSINK/THERMAL DISSIPATION TO MEET MANUFACTURER'S SPECIFICATIONS **DEPENDING ON DESIRED OUTPUT VOLTAGES, R10 MUST BE SPLIT INTO MULTIPLE RESISTORS, TO MEET MANUFACTURER'S VOLTAGE SPECIFICATION. ***C4 MUST BE SIZED TO MEET LARGE SIGNAL STABILITY CRITERIA DESCRIBED IN THE APPLICATIONS INFORMATION SECTION C1: 25V X5R OR X7R CERAMIC C2: 25V X5R OR X7R CERAMIC C3: 25V ELECTROLYTIC C5: TDK CKG57NX7R2J474M D1: VISHAY US1M 1000V M1: FAIRCHILD FQP34N20L R1 THRU R4, R6 THRU R9, R11: USE 1% 0805 R5: IRC LR SERIES 2512 RESISTORS R10A, R10B, R10C USE: 200V 1206 RESISTORS T1: COILCRAFT GA3459-AL
R8, 40.2k
C1 10F
TO MICRO R1, 475k
M1* VCC R5 6m R10**
VTRANS
R2, 69.8k OVLO1 R3, 475k UVLO2
VCC
R4, 69.8k
C6 10nF
R11
R9
Suggested Component Values
VOUT(V) 100 200 300 400 500 R9 (k) 3.32 1.65 1.10 0.825 Tie to GND R11(k) 0.383 0.768 1.13 1.54 1.74 R10(k) 30.9 124 274 499 715
VOUT AC COUPLED 2V/DIV VSW 50V/DIV IPRI 10A/DIV
Steady-State Operation with 1.1mA Load Current
Transformer primary inductance limits V OUT comparator operation to VOUT = 400VMAX. RVOUT and RBG should be tied to ground when operating VOUT above 400V.
10s/DIV
3751 TA03b
Regulation Efficiency vs Load Current (VOUT = 500V)
90 85 OUTPUT VOLTAGE (V) 510 EFFICIENCY (%) 80 75 70 65 60 VTRANS = 24V VTRANS = 12V VTRANS = 5V 0 50 100 150 OUTPUT VOLTAGE (V) 200
3751 TA03c
VOUT vs Load Current
515 VTRANS = 24V VTRANS = 12V VTRANS = 5V VOUT COUPLED 2V/DIV VSW 50V/DIV IPRI 10A/DIV
Steady-State Operation with 100mA Load Current
505
500
10s/DIV
3751 TA03e
495
0
50
100 ILOAD (mA)
150
200
3751 TA03d
3751f
26
LT3751 PACKAGE DESCRIPTION
UFD Package 20-Pin Plastic QFN (4mm x 5mm)
(Reference LTC DWG # 05-08-1711 Rev B)
0.70 0.05
4.50 0.05 1.50 REF 3.10 0.05
2.65 0.05 3.65 0.05
PACKAGE OUTLINE
0.25 0.05 0.50 BSC 2.50 REF 4.10 0.05 5.50 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 0.10 (2 SIDES) PIN 1 TOP MARK (NOTE 6) 0.75 0.05 R = 0.05 TYP 1.50 REF 19 20 0.40 0.10 PIN 1 NOTCH R = 0.20 OR C = 0.35
1 2 5.00 0.10 (2 SIDES) 2.50 REF 3.65 0.10 2.65 0.10
(UFD20) QFN 0506 REV B
0.200 REF 0.00 - 0.05
R = 0.115 TYP
0.25 0.05 0.50 BSC
BOTTOM VIEW--EXPOSED PAD NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X). 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
3751f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
27
LT3751 TYPICAL APPLICATION
300V Regulated Power Supply
VTRANS 24V T1 1:10 C2 5x 2.2F R6 40.2k RVTRANS CHARGE CLAMP VCC 24V VCC C1 10F TO MICRO R1 432k R2 475k R3 432k UVLO2 VCC R4 475k OVLO2 GND RBG
3751 TA05
D1
C3 680F R7 18.2k
* *
+
VOUT 300V 0mA TO 270mA C4 20F
OFF ON
RDCM RVOUT
DONE FAULT UVLO1
HVGATE LVGATE CSP
M1 VCC R5 6m
R8 274k
VTRANS
LT3751 OVLO1 CSN
FB R9 1.13k C5 10nF
C1: 25V X5R OR X7R CERAMIC CAPACITOR C2: 25V X5R OR X7R CERAMIC CAPACITOR C3: 25V ELECTROLYTIC C4: 330V RUBYCON PHOTOFLASH CAPACITOR D1: VISHAY USIM 1000V
M1: FAIRCHILD FQP34N20L R1 THROUGH R4: USE 1% 0805 RESISTORS R5: USE TWO PARALLEL 5m IRC LR SERIES 2512 RESISTORS T1: SUMIDA PS07-299, 20A TRANSFORMER
RELATED PARTS
PART NUMBER LTC3225 LT3420/LT3420-1 LT3468/LT3468-1 LT3468-2 LT3484-0/LT3484-1 LT3484-2 LT3485-0/LT3485-1 LT3485-2/LT3485-3 LT3585-0/LT3585-1 LT3585-2/LT3585-3 LT3750 DESCRIPTION 150mA Supercapacitor Charger 1.4A/1A, Photoflash Capacitor Charger with Automatic Top-Off 1.4A, 1A, 0.7A, Photoflash Capacitor Charger 1.4A, 0.7A, 1A Photoflash Capacitor Charger 1.4A, 0.7A, 1A, 2A Photoflash Capacitor Charger with Output Voltage Monitor and Integrated IGBT 1.2A, 0.55A, 0.85A, 1.7A Photoflash Capacitor Charger with Adjustable Input Current and IGBT Drivers Capacitor Charger Controller COMMENTS VIN: 2.75V to 5.5V, Charges Two Supercapacitors in Series to 4.8V or 5.3V Charges 220F to 320V in 3.7 Seconds from 5V, VIN: 2.2V to 16V, ISD < 1A, 10-Lead MS Package VIN: 2.5V to 16V, Charge Time: 4.6 Seconds for LT3468 (0V to 320V, 100F , VIN = 3.6V), ISD < 1A, ThinSOTTM Package , VIN: 1.8V to 16V, Charge Time: 4.6 Seconds for LT3484-0 (0V to 320V, 100F VIN = 3.6V), ISD < 1A, 2mm x 3mm 6-Lead DFN Package , VIN: 1.8V to 10V, Charge Time: 3.7 Seconds for LT3485-0 (0V to 320V, 100F VIN = 3.6V), ISD < 1A, 3mm x 3mm 10-Lead DFN Package , VIN: 1.5V to 16V, Charge Time: 3.3 Seconds for LT3585-3 (0V to 320V, 100F VIN = 3.6V), ISD < 1A, 3mm x 2mm DFN-10 Package VIN: 3V to 24V, Charge Time: 300ms for (0V to 300V, 100F) MSOP-10 Package
ThinSOT is a trademark of Linear Technology Corporation.
3751f
28 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 1108 * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2008

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