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EL7566
Data Sheet December 1, 2004 FN7102.5
Monolithic 6 Amp DC-DC Step-Down Regulator
The EL7566 is a full-feature synchronous step-down regulator capable of up to 6A and 96% efficiency. The device operates from 3V to 6V input supply (VIN). With internal CMOS power FETs, the device can operate at up to 100% duty ratio, allowing for an output voltage range of 0.8V to nearly VIN. An adjustable switching frequency up to 1MHz enables the use of small components, thereby reducing board area consumption to under 0.72sq-in on one side of a PCB. The EL7566 operates in constant frequency PWM mode, making external synchronization possible. A soft-start feature is integrated in the EL7566 to limit in-rush currents and allow for a smooth voltage ramp from zero to regulation. Other start-up features are integrated to add flexibility for synchronizing many supplies in multiple configurations. The EL7566 also offers a voltage margining capability that shifts the output voltage 5% for validation of system card performance and reliability during manufacturing tests. A junction temperature indicator conveniently monitors the silicon die temperature, saving time in thermal characterization. An easy-to-use simulation tool is available for download and can be used to modify design parameters such as switching frequency, voltage ripple, ambient temperature, as well as view schematics waveforms, efficiency graphs, and complete BOM with Gerber layout.
Features
* Integrated MOSFETs * 6A continuous output current * Up to 96% efficiency * Multiple supply start-up tracking * Built-in 5% voltage margining * 3V to 6V input voltage * 0.72 in2 footprint with components on one side of PCB * Adjustable switching frequency to 1MHz * Oscillator synchronization possible * 100% duty ratio * Junction temperature indicator * Over-temperature protection * Internal soft-start * Variable output voltage down to 0.8V * Power-good indicator * 28-pin HTSSOP package * Pb-Free Available (RoHS Compliant)
Applications
* Point-of-regulation power supplies * FPGA Core and I/O supplies * DSP, CPU Core, and IO supplies * Logic/Bus supplies * Portable equipment
Ordering Information
PART NUMBER EL7566DRE EL7566DRE-T7 EL7566DRE-T13 EL7566DREZ (Note) PACKAGE 28-HTSSOP 28-HTSSOP 28-HTSSOP 28-HTSSOP (Pb-free) TAPE & REEL 7" 13" 7" 13" PKG. DWG. # MDP0048 MDP0048 MDP0048 MDP0048 MDP0048 MDP0048 MDP0048 7" 13" MDP0048 MDP0048
Related Documentation
* Technical Brief 415 - Using the EL7566 Demo Board * Easy-to-use applications software simulation tool available at www.intersil.com/dc-dc
EL7566DREZ-T7 (Note) 28-HTSSOP (Pb-free) EL7566DREZ-T13 (Note) 28-HTSSOP (Pb-free) EL7566AIREZ (Note) 28-HTSSOP (Pb-free) -40C to 85C
EL7566AIREZ-T7 (Note) 28-HTSSP (Pb-free) -40C to 85C EL7566AIREZ-T13 (Note) 28-HTSSOP (Pb-free) -40C to 85C
NOTE: Intersil Pb-free products employ special Pb-free material sets; molding compounds/die attach materials and 100% matte tin plate termination finish, which are RoHS compliant and compatible with both SnPb and Pb-free soldering operations. Intersil Pb-free products are MSL classified at Pb-free peak reflow temperatures that meet or exceed the Pb-free requirements of IPC/JEDEC J STD-020C.
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CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 321-724-7143 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright (c) Intersil Americas Inc. 2004. All Rights Reserved. All other trademarks mentioned are the property of their respective owners.
EL7566 Typical Application Diagram
CC R2 10K R1 21.5K 8200pF 0.047F 3 FB 4 VO 5 VTJ 6 TM 7 SEL 8 LX 2.7H VOUT (2.5V, 6A) 150F 9 LX 10 LX 11 LX 12 LX 13 LX 14 NC STN 26 STP 25 EN 24 PG 23 VDD 22 VIN 21 VIN 20 VIN 19 PGND 18 PGND 17 PGND 16 NC 15 VIN (3V TO 6V) 100F 0.22F RC 10K 2 VREF COSC 27 1 COMP SGND 28 270pF
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EL7566
Absolute Maximum Ratings (TA = 25C)
VIN, VDD to SGND. . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +6.5V VX to PGND. . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to VIN +0.3V SGND to PGND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +0.3V COMP, VREF, FB, VO, VTJ, TM, SEL, PG, EN, STP, STN, COSC to SGND . . . . . -0.3V to VDD +0.3V Storage Temperature . . . . . . . . . . . . . . . . . . . . . . . .-65C to +150C Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +135C Operating Ambient Temperature DRE . . . . . . . . . . . . . 0C to +85C Operating Ambient Temperatute AIRE . . . . . . . . . . .-40C to +85C
CAUTION: Stresses above those listed in "Absolute Maximum Ratings" may cause permanent damage to the device. This is a stress only rating and operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied. IMPORTANT NOTE: All parameters having Min/Max specifications are guaranteed. Typ values are for information purposes only. Unless otherwise noted, all tests are at the specified temperature and are pulsed tests, therefore: TJ = TC = TA
DC Electrical Specifications
PARAMETER VIN VREF VREFTC VREFLOAD VRAMP IOSC_CHG IOSC_DIS IVDD IVDD_OFF VDD_OFF VDD_ON TOT THYS ILEAK ILMAX RDSON1 RDSONTC2 RDSONTC ISTP ISTN VPGP VPGN VPG_HI VPG_LO VOVP VFB VFB_LINE GMEA VFB_TC FS IFB Input Voltage Range Reference Accuracy
VDD = VIN = 3.3V, TA = TJ = 25C, COSC = 390pF, Unless Otherwise Specified CONDITIONS MIN 3 1.24 1.26 50 0 < IREF < 50A -1 1.15 0.1V < VOSC < 1.25V 0.1V < VOSC < 1.25V VEN = 1 (L disconnected) EN = 0 2.4 2.6 135 20 EN = 0, LX = 6V (low FET), LX = 0V (high FET) 7.8 29 25 0.2 VSTP = VIN/2 VSTN = VIN/2 With respect to target output voltage With respect to target output voltage IPG = 1mA IPG = -1mA 10 ILOAD = 0A VIN = 3.3V, VIN = 10%, ILOAD = 0A VCC = 0.65V 0C < TA < 85C, ILOAD = 3A 300 VFB = 0V 85 0.79 0.8 0.2 125 1 370 100 440 200 0.81 0.5 165 6 -14 2.6 0.5 -4 2.5 2.5 4 14 -6 50 10 2 200 8 2.7 1 5 1.5 2.65 2.95 TYP MAX 6 1.28 UNIT V V ppm/C % V A mA mA mA V V C C A A m m m/C A A % % V V % V % s % kHz nA
DESCRIPTION
Reference Temperature Coefficient Reference Load Regulation Oscillator Ramp Amplitude Oscillator Charge Current Oscillator Discharge Current VDD Supply Current VDD Standby Current VDD for Shutdown VDD for Startup Over-temperature Threshold Over-temperature Hysteresis Internal FET Leakage Current Peak Current Limit PMOS On Resistance NMOS On Resistance RDSON Tempco STP Pin Input Pull-down Current STN Pin Input Pull-up Current Positive Power Good Threshold Negative Power Good Threshold Power Good Drive High Power Good Drive Low Output Overvoltage Protection Output Initial Accuracy Output Line Regulation Error Amplifier Transconductance Output Temperature Stability Switching Frequency Feedback Input Pull-up Current
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DC Electrical Specifications
PARAMETER VEN_HI VEN_LO IEN TM, SEL_HI TM, SEL_LO VDD = VIN = 3.3V, TA = TJ = 25C, COSC = 390pF, Unless Otherwise Specified (Continued) CONDITIONS MIN 2.6 1 VEN = 0 -4 2.6 1 -2.5 TYP MAX UNIT V V A V V
DESCRIPTION EN Input High Threshold EN Input Low Threshold Enable Pull-up Current Input High Level Input Low Level
Pin Descriptions
PIN NUMBER 1 2 3 4 5 6 7 8, 9, 10, 11, 12, 13 14, 15 16, 17, 18 19, 20, 21 22 23 24 25 26 27 28 PIN NAME COMP VREF FB VO VTJ TM SEL LX NC PGND VIN VDD PG EN STP STN COSC SGND PIN FUNCTION Error amplifier output; place loop compensation components here Bandgap reference bypass capacitor; typically 0.022F to 0.047F to SGND Voltage feedback input; connected to external resistor divider between VOUT and SGND for adjustable output; also used for speed-up capacitor connection Output sense for fixed output option. This pin can be open for EL7566 Junction temperature monitor output Stress test enable; allows 5% output movement; connect to SGND if function is not used Positive or negative stress select; see text Inductor drive pin; high current output whose average voltage equals the regulator output voltage Not used Ground return of the regulator; connected to the source of the low-side synchronous NMOS Power FET Power supply input of the regulator; connected to the drain of the high-side PMOS Power FET Control circuit positive supply; connected to VIN through an internal 20 resistor Power-good window comparator output; logic 1 when regulator output is within 10% of target output voltage Chip enable, active high; a 2.5A internal pull-up current enables the device if the pin is left open; a capacitor can be added at this pin to delay the start of a converter Auxilliary supply tracking positive input; tied to regulator output to synchronize start-up with a second supply; leave open for standalone operation; 2A internal pull-up current Auxiliary supply tracking negative input; connect to output of a second supply to synchronize start-up; leave open for standalone operation; 2A internal pull-up current Oscillator timing capacitor (see performance curves) Control circuit negative supply or signal ground
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EL7566 Block Diagram
TM SEL VREF VTJ 2.2nF EN VDD JUNCTION TEMPERATURE VOLTAGE REFERENCE 0.047F COSC 390pF
OSCILLATOR VDD 20 VIN
0.22F VIN 100F 2.7H VOUT (2.5V, 6A) 150F PGND
STP STN
POWER TRACKING PWM CONTROLLER DRIVERS
POWER FET POWER FET
EA CURRENT SENSE COMP RC CC SGND FB R2 R1 VREF + VO VDD PG
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FN7102.5 December 1, 2004
EL7566 Typical Performance Curves
VIN = VD = 5V, VO = 2.5V, IO = 6A, fS = 500kHz, L = 2.7H, CIN = 100F, COUT = 150F, TA = 25C unless otherwise noted.
100 95 EFFICIENCY (%) 90 85 80 75 70 65 60 0 1 2 3 IO (A) 4 5 6 VO=0.8V VO=1V VO=1.2V VO=1.8V VO=3.3V VO=2.5V EFFICIENCY (%)
100 95 90 85 80 75 70 65 60 0 1 2 3 IO (A) 4 5 6 VO=1V VO=1.2V VO=0.8V VO=2.5V VO=1.8V
FIGURE 1. EFFICIENCY (VIN = 5V)
FIGURE 2. EFFICIENCY (VIN = 3.3V)
1.265 1.26 1.255 VTJ VDD=5V 1.25 1.24 1.245 VDD=3.3V
1.6 1.5 1.4
VREF
1.3 1.2 1.1 1 VDD=5V VDD=3.3V
0
50
100
150
0
0
50
100
150
JUNCTION TEMPERATURE (C)
JUNCTION TEMPERATURE (C)
FIGURE 3. VREF vs TEMPERATURE
FIGURE 4. VTJ vs TEMPERATURE
4 3.5 3 2.5 2 1.5 1 VEN_LOW VEN_HI FS (kHz)
1200 1000 800 600 500 200 0 100 VDD=3.3V VDD=5V
3
3.5
4
4.5 VDD (V)
5
5.5
6
200
300
400 COSC (pF)
500
600
700
FIGURE 5. VEN_HI & VEN_LOW vs VDD
FIGURE 6. FS vs COSC
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EL7566 Typical Performance Curves
VIN = VD = 5V, VO = 2.5V, IO = 6A, fS = 500kHz, L = 2.7H, CIN = 100F, COUT = 150F, TA = 25C unless otherwise noted. (Continued)
526 524 SWITCHING FREQUENCY 522 520 518 514 512 510 508 506 504 0 1 2 3 IO (A) 4 5 6 VIN=3.3V (%) 516 VIN=5V
0.1 0.05 0 -0.05 -0.1 -0.15 -0.2 -0.25 -0.3 -0.35 0 1 2 3 IO (A) 4 5 6
FIGURE 7. FS vs LOAD CURRENT
FIGURE 8. LOAD REGULATIONS
JEDEC JESD51-3 LOW EFFECTIVE THERMAL CONDUCTIVITY TEST BOARD HTSSOP EXPOSED DIEPAD NOT SOLDERED TO PCB
50 45 JA (C/W) 40 35 30 25
POWER DISSIPATION (W)
CONDITION: 28-Pin HTSSOP THERMAL PAD SOLDERED TO 2-LAYER PCB WITH 0.039" THICKNESS AND 1 OZ. COPPER ON BOTH SIDES
1.2 1 0.8 0.6 0.4 0.2 0 1.136W
HT S 11 0
JA =
P2 C 8 /W
SO
1
2
3
4
5
6
7
8
9
0
25
50
75 85 100
125
150
PCB AREA (in2)
AMBIENT TEMPERATURE (C)
FIGURE 9. HTSSOP THERMAL RESISTANCE vs PCB AREA (NO AIR FLOW)
FIGURE 10. PACKAGE POWER DISSIPATION vs AMBIENT TEMPERATURE
JEDEC JESD51-7 HIGH EFFECTIVE THERMAL CONDUCTIVITY TEST BOARD HTSSOP EXPOSED DIEPAD SOLDERED TO PCB PER JESD51-5 4.5 POWER DISSIPATION (W) 4 3.5 3 2.5 2 1.5 1 0.5 0 0 25 50 75 85 100 125 150 4.167W
JA
HT SS O P2 =3 0 8 C/ W
AMBIENT TEMPERATURE (C)
FIGURE 11. PACKAGE POWER DISSIPATION vs AMBIENT TEMPERATURE
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FN7102.5 December 1, 2004
EL7566 Waveforms
VIN = VD = 5V, VO = 2.5V, IO = 6A, fS = 500kHz, L = 2.7H, CIN = 100F, COUT = 150F, TA = 25C unless otherwise noted.
VIN (5V/DIV) IIN (2A/DIV) VO (2V/DIV)
VIN (200mV/DIV) IL (2A/DIV)
PG
VLX (5V/DIV)
VO (50mV/DIV) 0.5ms/DIV 1s/DIV
FIGURE 12. START-UP
FIGURE 13. STEADY-STATE OPERATION
4.5A VEN IO 1.5A VO (100mV/DIV)
IIN (2A/DIV)
VO (2V/DIV) 50s/DIV
100s/DIV
FIGURE 14. SHUT-DOWN
FIGURE 15. TRANSIENT RESPONSE
TM
PG VO (2V/dIv)
SEL
VO (200mV/DIV)
VLX (5V/DIV)
1ms/DIV
0.5ms/DIV
FIGURE 16. VOLTAGE MARGINING
FIGURE 17. OVERVOLTAGE SHUT-DOWN
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EL7566 Waveforms
VIN = VD = 5V, VO = 2.5V, IO = 6A, fS = 500kHz, L = 2.7H, CIN = 100F, COUT = 150F, TA = 25C unless otherwise noted. (Continued)
VIN (5V/DIV) IIN (2A/DIV) VO (2V/DIV)
VIN (5V/DIV)
VO1=2.5V VO2=1.8V
PG
5ms/DIV
5ms/DIV
FIGURE 18. ADJUSTABLE START-UP
FIGURE 19. TRACKING START-UP
Detailed Description
The EL7566 is a 6A capable buck regulator operating from an input voltage range of 3V to 6V. The duty cycle can be adjusted from 0% to 100% allowing for a wide range of programmable output voltages. Patented on-chip resistorless current-sensing enables current mode control for excellent step load response. Overcurrent, Overvoltage, input Undervoltage, and thermal protection is integrated along with soft-start and power-up sequencing features to produce an overall robust power solution for general purpose applications. placed from the EN pin to GND to program a delay between when the rising POR threshold for VIN is met and when softstart begins. The programmable delay time, TD, is governed by Equation 1.
V EN_HI T D = C EN x ------------------I
EN
where: * CEN is the capacitance at EN pin * VEN_HI is the EN input high level (function of VDD voltage, see Figure 5) * IEN is the EN pin pull-up current, nominal 2.5A
EL7566DRE vs. EL7566AIRE
The EL7566AIRE includes the following feature changes from the EL7566DRE: * Up to 6A Current Sinking Capability * Expanded Temperature Range: -40oC to 85oC * No Overvoltage Protection
Steady-State Operation
Under all steady-state conditions the converter will operate in fixed frequency continuous-conduction mode. For fast transient response and ease of controllability, a peak current-mode control method is employed. The inductor current is sensed from the upper PMOS. This current signal serves as the ramp to the PWM comparator and is compared against the difference signal generated by the transconductance error amplifier. Slope compensation for the ramp is used to allow for 100% duty cycle operation (see Figure 20). The pulse-width modulated square wave output of the PWM comparator is amplified and serves as the gate drive signals for the switching power FETs. 100% DUTY RATIO EL7566 uses CMOS as internal synchronous power switches. The upper and lower switches are PMOS and NMOS respectively. The upper PMOS saves the need for a boot capacitor normally seen in NMOS/NMOS half-bridges.
Start-Up
The EL7566 employs a digital soft-start feature to suppress the in-rush current needed to charge the output capacitance and smoothly ramp the output voltage to regulation (See Figure 12). The normal start-up process begins when the input voltage reaches the rising POR threshold (~2.8V) and EN pin is transitioned HIGH by an internal 2.5A current source. The output voltage is then digitally ramped to regulation over a 2ms period. The 2ms soft start-up time can be extended if needed by configuring the STP and STN pins. (refer to Full Start-Up Control section). If the input voltage is ramped slowly, soft-start may be initiated before the input supply has reached regulation. The lower input voltage will have increased current demand during start-up and may risk an overcurrent event. To prevent such an event from occurring, a capacitor can be 9
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It also allows 100% turn-on of the upper PMOS switch, achieving VO close to VIN. The maximum achievable VO is:
V O = V IN - ( R L + R DSON1 ) x I O
OSC pin to GND (COSC). The triangle waveform has 95% duty ratio and runs from 0.2V to 1.2V. Refer to the curve in Figure 6 for the appropriate value of COSC for the desired frequency. If external synchronization is desired, the circuit in Figure 21 can be used.
100pF EL7566 EXTERNAL SYNC SOURCE
Where RL is the DC resistance on the inductor and RDSON1 is the PMOS on-resistance, nominally 30m at room temperature with a temperature coefficient of 0.2m/C. OUTPUT VOLTAGE SELECTION The output voltage can be as high as the input voltage minus the PMOS and inductor voltage drops (as seen previously in Equation 2). Referring to the Typical Application Circuit on page 2, use R1 and R2 to set the output voltage according to the following formula:
R 1 V O = 0.8 x 1 + ------ R 2
COSC
FIGURE 20. EXTERNAL SYNC CIRCUIT
Always choose the converter self-switching frequency 20% lower than the sync frequency to accommodate component variations.
Protection Features
The EL7566 features a wide range of protective measures to prevent the persistence of damaging system conditions. These features are overvoltage, overcurrent, Power-OnReset (POR), and Thermal Shutdown protection. OVERVOLTAGE PROTECTION (OVP) The EL7566 monitors the output voltage and will shut down if it exceeds 110% of the set regulation point. This is accomplished by comparing the reference to the FB pin voltage. If an overvoltage condition is met, the controller will turn the high-side switch off, the low-side switch on, and pull PGOOD low. The converter will not latch off and will proceed with a soft-start as soon as the fault condition is cleared. OVERCURRENT PROTECTION (OCP) The current information for PWM ramp generation is also used for overcurrent protection. The measured current is compared against a preset Overcurrent threshold (~7-10A). If the output current exceeds the threshold, the output will shut down by turning off the high-side switch and turning the low-side switch on. This event, like OVP, will not latch the converter off. A soft-start will be initiated when the fault is cleared. POWER-ON RESET (POR) To ensure proper regulator operation, a power-on reset feature monitors the input voltage. When adequate input voltage is achieved (VDD > 2.8V), the converter is allowed to soft-start. However, if VDD falls below 2.5V, the regulator will shut down in the same manner as OVP or OCP. THERMAL PROTECTION AND JUNCTION TEMPERATURE INDICATOR An internal temperature sensor continuously monitors the junction temperature. If the junction temperature exceeds 135C, the regulator is in a fault condition and will shut down. When the temperature falls back below 110C, the regulator goes through the soft-start procedure again.
Some standard values of R1 and R2 are listed in Table 1.
TABLE 1. VO (V) 0.8 1 1.2 1.5 1.8 2.5 3.3 R1 (k) 2 2.49 4.99 10 12.7 21.5 36 R2 (k) Open 10 10 11.5 10.2 10 11.5
It is important that the series combination of R1 and R2 is large enough as to not draw excessive current from the output. VOLTAGE MARGINING The EL7566 has built-in 5% load stress test (commonly called voltage margining) function. Combinations of TM and SEL set the margins shown in Table 2. When this function is not used, both pins should be connected to SGND, either directly or through a 10k resister. Figure 16 shows this feature.
TABLE 2. CONDITION Normal High Margin Low Margin TM 0 1 1 SEL X 1 0 VO Nominal Nominal + 5% Nominal - 5%
SWITCHING FREQUENCY The regulator has a programmable switching frequency of 200kHz to 1MHz. The switching frequency is generated by a relaxation comparator and adjusted by a capacitor from the 10
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The VTJ pin reports a voltage proportional to the junction temperature. Equation 3 illustrates the relationship and can be used to accurately evaluate thermal design points.
1.2 - V TJ T J = 75 + -----------------------0.00384
LINEAR START-UP In the linear start-up tracking configuration, the regulator with lower output voltage, VO2, tracks the one with higher output voltage, VO1.
C
Full Start-Up Control
The EL7566 offers full start-up control. The core of this control is a start-up comparator in front of the main PWM controller. The STP and STN are the inputs to the comparator, whose HI output forces the PWM comparator to skip switching cycles. The user can choose any of the following control configurations: ADJUSTABLE SOFT-START In this configuration, the ramp-up time is adjustable to any time longer than the building soft-start time of 2ms. The approximate ramp-up time, TST, is:
VO T ST = RC --------- V IN
VO2
+ VO1 EL7566 VIN
+
STN STP
R VIN
EL7566
VO1 VO2
FIGURE 23. LINEAR START-UP TRACKING
OFFSET START-UP Compared with the cascade start-up, this configuration allows Regulator 2 to begin the start-up process when VO1 reaches a particular value of VREF*(1+RB/RA) before PG goes HI, where VREF is the regulator reference voltage. VREF=1.26.
+ VO EL7566
STN STP
C R 200K VIN
0.1F VO
VREF TST VO2
+ VIN
RB RA
FIGURE 21. ADJUSTABLE START-UP
EL7566
VO1 EL7566 VIN
CASCADE START-UP In this configuration, EN pin of Regulator 2 is connected to the PG pin of Regulator 1 (Figure 22). VO2 will only start after VO1 is good.
VREF(1+RB/RA) VO1 VO2
FIGURE 24. OFFSET START-UP TRACKING
EN VO2 EL7566 VO1 EL7566 PG VIN
Component Selection
INPUT CAPACITOR The main functions of the input capacitor(s) are to maintain the input voltage steady and to filter out the pulse current passing through the upper switch. The root-mean-square value of this current is:
V O x ( V IN - V O ) I IN,RMS = ----------------------------------------------- x I O 1/2 ( I O ) V IN
VO1 VO2
FIGURE 22. CASCADE START-UP
for a wide range of VIN and VO. For long-term reliability, the input capacitor or combination of capacitors must have the current rating higher than IIN,RMS. Use X5R or X7R type ceramic capacitors, or SPCAP or POSCAP types of Polymer capacitors for their high current handling capability. 11
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INDUCTOR The NMOSNMOS reverse current limit is set at about 0.5A. For optimal operation, the peak-to-peak inductor current ripple IL should be less than 1A. The following equation gives the inductance value:
( V IN - V O ) x V O L = ------------------------------------------V IN x I L x F S
where: * GMPWM is the transconductance of the PWM comparator, GMPWM = 120S
VO R OUT = ------IO
* ESR is the ESR of the output capacitor * COUT is output capacitance * GMEA is the transconductance of the error amplifier, GMEA = 120S * FC is the intended crossover frequency of the loop. For best performance, set this value to about one-tenth of the switching frequency. * Once RC is chosen, CC is decided by:
R OUT C C = 1.5 x C OUT x --------------RC
The peak current the inductor sees is:
I L I LPK = I O + -------2
When inductor is chosen, it must be rated to handle the peak current and the average current of IO. OUTPUT CAPACITOR Output voltage ripple and transient response are the predominant factors when choosing the output capacitor. Initially, output capacitance should be sized with an ESR to satisfy the output ripple VO requirement:
V O = I L x ESR
Design Example A 5V to 2.5V converter with a 6A load requirement. 1. Choose the input capacitor The input capacitor or combination of capacitors has to be able to take about 1/2 of the output current, e.g., 3A. Panasonic EEFUD0J101XR is rated at 3.3A, 6.3V, meeting the above criteria. 2. Choose the inductor. Set the converter switching frequency at 500kHz:
( V IN - V O ) x V O L = ------------------------------------------V IN x I L x F S
When a step load change, IO, is applied to the converter, the initial voltage drop can be approximated by ESR*IO. The output voltage will continue to drop until the control loop begins to correct the output voltage error. Increasing the output capacitance will lessen the impact of load steps on output voltage. Increasing loop bandwidth will also reduce output voltage deviation under step load conditions. Some experimentation with converter bandwidth and output filtering will be necessary to generate a good transient response (Reference Figure 15). As with the input capacitor, it is recommended to use X5R or X7R type of ceramic capacitors. SPCAP or POSCAP type Polymer capacitors can also be used for the low ESR and high capacitance requirements of these converters. Generally, the AC current rating of the output capacitor is not a concern because the RMS current is only 1/8 of IL. LOOP COMPENSATION Current-mode control in system forces the inductor current to be proportional to the error signal. This has the advantage of eliminating the double pole response of the output filter, and reducing complexity in the overall loop compensation. A simple Type 1 compensator is adequate to generate a stable, high-bandwidth converter. The compensation resister is decided by:
F C x 2 x x ( ESR + R OUT ) x C OUT IO R C = ----------- x ------------------------------------------------------------------------------------------------VFB GM PWM x GM EA
IL = 1A yields 2.3H. Leave some margin and choose L = 2.7H. Coilcraft's DO3316P-272HC has the required current rating. 3. Choose the output capacitor L = 2.7H yields about 1A inductor ripple current. If 25mV of ripple is desired, COUT's ESR needs to be less than 25m. Panasonic's EEFUD0G151XR 150F has an ESR of 12m and is rated at 4V. ESR is not the only factor deciding the output capacitance. As discussed earlier, output voltage droops less with more capacitance when converter is in load transient. Multiple iterations may be needed before final components are chosen. 4. Loop compensation 50kHz is the intended crossover frequency. With the conditions RC and CC are calculated as: RC = 10.5k and CC = 8900pF, round to standard value of 8200pF.
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For convenience, Table 3 lists the compensation values for frequently used output voltages.
TABLE 3. COMPENSATION VALUES VO (V) 3.3 2.5 1.8 1.5 1.2 1 0.8 RC (K) 13.7 10.5 7.68 6.49 5.23 4.42 3.57 CC (PF) 8200 8200 8200 8200 8200 8200 8200
Layout Considerations
The layout is very important for the converter to function properly. Follow these tips for best performance: 1. Separate the Power Ground ( ) and Signal Ground ( ); connect them only at one point right at the SGND pin 2. Place the input capacitor(s) as close to VIN and PGND pins as possible 3. Make as small as possible the loop from LX pins to L to CO to PGND pins 4. Place R1 and R2 pins as close to the FB pin as possible 5. Maximize the copper area around the PGND pins; do not place thermal relief around them 6. Thermal pad should be soldered to PCB. Place several via holes under the chip to the ground plane to help heat dissipation The demo board is a good example of layout based on this outline. Please refer to the EL7566 Application Brief.
Thermal Management
The EL7566 is packaged in a thermally-efficient HTSSOP-28 package, which utilizes the exposed thermal pad at the bottom to spread heat through PCB metal. Therefore: 1. The thermal pad must be soldered to the PCB. 2. Maximize the PCB area. 3. If a multiple layer PCB is used, thermal vias (13 to 25 mil) must be placed underneath the thermal pad to connect to ground plane(s). Do not place thermal reliefs on the vias. Figure 25 shows a typical connection. The thermal resistance for this package is as low as 26C/W for 2 layer PCB of 0.39" thickness (See Figure 9). The actual junction temperature can be measured at VTJ pin. The thermal performance of the IC is heavily dependent on the layout of the PCB. The user should exercise care during the design phase to ensure the IC will operate within the recommended environmental conditions.
COMPONENT SIDE CONNECTION
GROUND PLANE CONNECTION
FIGURE 25. PCB LAYOUT - 28-PIN HTSSOP PACKAGE
13
FN7102.5 December 1, 2004
EL7566 Package Outline Drawing
(R)
1. The package drawing shown here may not be the latest version. To check the latest revision, please refer to the Intersil website at http://www.intersil.com/design/packages/index.asp
All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems. Intersil Corporation's quality certifications can be viewed at www.intersil.com/design/quality
Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
For information regarding Intersil Corporation and its products, see www.intersil.com 14
FN7102.5 December 1, 2004

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