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LOW VOLTAGE, BOOST DC-DC CONTROLLER
August 22, 2000
SC1408
TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com
DESCRIPTION
The SC1408 is a low voltage boost controller that operates from a 1.8V to 16.5V input range. A shutdown pin allows the user to turn the controller off reducing supply current to less than 2A typical. Output voltage can be preset to 5V or is adjustable from 3V to 16.5V with a resistor divider. The controller changes frequency in low load conditions to improve efficiency. The SC1408 was designed for two cell Alkaline or single cell Lithium Ion battery applications. With the proper external components it can be used as a boost converter or a buck/boost converter. A current sense is implemented with an external resistor that is not in the load current path. The SC1408 operates in "bootstrapped" mode. When the input voltage to the device is less than 2.5V during startup, a low voltage 50% fixed duty cycle oscillator is switched in to initiate boost action.
FEATURES *= 1.8V to 16.5V input range *= Up to 10W output power *= Preset 5V or adjustable output *= Up to 300kHz switching frequency *= 10A max shutdown current *= Industrial temperature range *= SO-8 or MSOP-8 package APPLICATIONS *= PDA Power supplies *= Battery powered applications *= Positive LCD Bias generator *= Portable communications (cellular phones) *= Peripheral card supplies *= Industrial power supplies ORDERING INFORMATION
Part Number
(1)
Package SO-8 MSOP-8
Temp. Range (TA) -40 to +85C
SC1408IS.TR SC1408IMS.TR
Note: (1) Only available in tape and reel packaging. A reel contains 2500 devices.
TYPICAL APPLICATION - BOOST CONFIGURATION
Vin (1.8V to 16.5V)
L1
D1
Vout (3V to 16.5V)
U1 2 4 + 5 C2 7
BST GATE
Q1 1 8
R3
SHDN ISENSE REF GND FB AGND
+ 3 6 R2 R4
C3
SC1408IS C5
1 (c) 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
LOW VOLTAGE, BOOST DC-DC CONTROLLER
August 22, 2000
SC1408
ABSOLUTE MAXIMUM RATINGS
Parameter Input Voltage Small Signal Ground to Power Ground GATE to GND FB, SHDN, REF, ISENSE to GND Operating Temperature Junction Temperature Range Storage Temperature Lead Temperature (Soldering) 10 seconds Thermal Resistance, Junction to Ambient SO-8 MSOP-8 Thermal Resistance, Junction to Case SO-8/MSOP-8 TA TJ TSTG TL =JA =JC
(1)
Symbol BST to GND GND to AGND
Maximum -0.3 to 18 0.1 -0.3 to VBST+0.3 -0.3 to min. of VBST+0.3 or 5 -40 to +85 -40 to +150 -65 to +160 +300 165 206 40
Units V V V V C C C C C/W
C/W
ELECTRICAL CHARACTERISTICS
PARAMETER Input Voltage SYM
Unless specified: VOUT = 5V; ILOAD = 0mA; TA = +25C CONDITIONS TA = 25C TA = -40C to +85C Supply Current VOUT = 16.5V, SHDN =0.4V VOUT = 10V, 1.6V =SHDN =5V Output Voltage Load Regulation Line Regulation Minimum Start Up Voltage Maximum Switch On Time Minimum Switch Off Time Efficiency Reference Voltage Reference Load Regulation Reference Line Regulation FB Trip Point Voltage FB Input Current VFB IFM VREF TON TOFF VIN = 4V, VOUT = 5V, ILOAD = 500mA IREF = 0A TA = -40C to +85C VIN = 2.0V to 5.0V, TA = -40C to +85C TA = -40C to +85C TA = -40C to +85C 4.800 MIN 1.8 1.8 110 2 5.0 60 7 1.8 9.6 1.4 16 2.3 87 1.176 1.200 1.224 -4 40 1.200 1.176 -4 +40 2 (c) 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 1.224 nA 10 100 22.4 3.2 TYP MAX UNITS 16.5 16.5 140 10 5.200 V V uA A V mV/A mV/V V s s % V mV V/V V
VIN = 2.0V, VOUT = 5V, ILOAD = 0mA to 500mA VIN = 2.7V to 4.0V, VOUT= 5V, ILOAD = 500mA No load
0A < IREF < 100A 5V < VOUT < 16.5V TA = 25C TA = -40C to +85C TA = 25C TA = -40C to +85C
LOW VOLTAGE, BOOST DC-DC CONTROLLER
August 22, 2000
SC1408
ELECTRICAL CHARACTERISTICS
(1)
Unless specified: VOUT = 5V; ILOAD = 0mA; TA = +25C PARAMETER SHDN Input High Voltage SHDN Input Low Voltage SHDN Input Current Current Limit Trip Level ISENSE Input Current GATE Rise Time GATE Fall Time GATE On Resistance SYM VIH VIL IIN VCS ISENSE VOUT = 5V, 1nF from GATE to GND VOUT = 5V, 1nF from GATE to GND GATE = high or low CONDITIONS VOUT = 2.7V to 16.5V VOUT = 2.7V to 16.5V VOUT = 16.5V, SHDN = 0V or 5V VOUT = 3V to 16.5V TA = 25C TA = -40C to +85C 85 80 100 0.01 50 50 15 30 MIN 1.6 0.4 +1 115 120 +1 TYP MAX UNITS V V A mV A ns
NOTE:
(1) This device is ESD sensitive. Use of standard ESD handling precautions is required.
3 (c) 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
LOW VOLTAGE, BOOST DC-DC CONTROLLER
August 22, 2000
SC1408
BLOCK DIAGRAM
REF
FB
1.20V REFERENCE ERROR COMP
MODE DETECT
+
INTERNAL BIAS
+ -
50mV BIAS
SHDN
MIN OFF TIME ONE SHOT START UP COMP
Q
TRIG 2.3uS
+ 2.5V
BST
S Q TRIG Q 16uS MAX ON TIME ONE SHOT R
GATE GND
LOW VOLTAGE OSCILLATOR
CURRENT SENSE AMP
+ 0.1V
ISENSE
AGND
PIN CONFIGURATION
Top View
PIN DESCRIPTION
Pin # 1 2 3 4 5 6 Pin Name GATE BST FB SHDN REF AGND GND ISENSE Pin Function Gate drive output. Supply voltage. Voltage feedback. Logic high shuts down the converter. Reference output pin. Small signal analog and digital ground. Power ground. Current sense pin.
(SO-8/MSOP-8)
7 8
4 (c) 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
LOW VOLTAGE, BOOST DC-DC CONTROLLER
August 22, 2000
SC1408
Fig. 1: Typical Application - Boost Configuration
Vin=3.3V
L1
22uH
D1 B130T R3 31.6k
Vout=5V
COILCRAFT DO3316P-223 U1 2 C2 100uF + 4 5 7 BST SHDN REF GND SC1408IS GATE ISENSE FB AGND 1 8 3
Q1 IRLR024N
C9 (1)
+
C8 100uF
+ 6 R2 0.05Ohm R4 10.0k C3 100uF
C5 0.1uF
(1) See Component Selection
Fig. 2: Typical Application - Buck/Boost (SEPIC) Configuration
D2 1N4148
L1A 22uH Vin=2.7V to 6V 2 1
C1 100uF +
D1 B130T
Vout=5V
U1 2 C2 100uF + C4 0.1uF C5 0.1uF 4 5 7 BST SHDN REF GND SC1408IS GATE ISENSE FB AGND 1 8 3 Q1 IRLL3303
R3 31.6k C9 (1) + C8 100uF
+ 6 R2 0.05Ohm 4 L1B 22uH R4 10.0k C3 100uF
3 L1A/L1B IS COUPLED INDUCTOR PULSE PE-53718 OR EQUIVALENT (1) See Component Selection
5 (c) 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
LOW VOLTAGE, BOOST DC-DC CONTROLLER
August 22, 2000
SC1408
Fig. 3: Achieving output voltages greater than 16.5V
D4 1N4148 L1 3.3V IN 22uH D1 25V OUT B130T U1 2 C4 0.1uF C2 100uF + 4 5 7 BST SHDN REF GND SC1408IS GATE ISENSE FB AGND 1 8 3 + 6 R2 0.05Ohm R4 10.0k C3 100uF R3 200k Q1 IRL3103S + C8 100uF R6 470
D3 12V
C5 0.1uF
Fig. 4: Implementing shutdown with input/output isolation
Q2 Si2301DS 3.3V IN R5 5.1k U1 2 C4 0.1uF J1 SHORT = RUN OPEN = SHUTDOWN 1 2 C5 0.1uF C2 100uF + 4 5 7 BST
L1
22uH
D1 12V OUT B130T R3 90k Q1 IRLL3303
GATE ISENSE FB AGND
1 8 3
+
SHDN REF GND SC1408IS
C8 100uF
+ 6 R2 0.05Ohm R4 10.0k C3 100uF
6 (c) 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
LOW VOLTAGE, BOOST DC-DC CONTROLLER
August 22, 2000
SC1408
THEORY OF OPERATION
The SC1408 is a modified hysteretic boost converter controller. The power switch is turned on when the output voltage falls slightly below it's setpoint. It remains on for approximately 16s, or until the inductor current aeV o reaches limit, whichever occurs first. The power switch is R 3 = R 4c O - 1 cV then turned off for 2.3s, or until the output voltage once e REF again falls below setpoint, whichever occurs last. Inductor The SC1408 is normally powered from the output voltThe SC1408 will work with a wide range of inductor valage. Internal circuitry, such as the bandgap, comparators and one shots will not function properly until the BST ues. A good choice for most applications is 22H. pin voltage reaches 2.5V. To ensure startup at low input Smaller inductor values result in higher peak currents and increased output ripple, while larger values will revoltages, the normal control circuitry is disabled and a special, low voltage start up oscillator generates an ap- sult in slower loop response. proximate square wave at the GATE pin, initiating boost action. When the output voltage reaches 2.5V, the nor- Transistor Selection mal control circuitry is enabled and the start up oscillator Normally the power switch will be an N-channel MOSFET, although in certain circumstances an NPN bipolar shuts down. may be substituted. To conserve power, a SHDN pin is provided which, when pulled high, shuts down most internal circuitry. The The choice of FET can be critical, especially in battery output voltage will then be one diode drop below the in- powered applications where the converter must be able to use all of the available energy in the battery. This reput. quires that the converter be capable of starting up from COMPONENT SELECTION very low input voltages. For example a two cell alkaline system's terminal voltage will drop to 1.8V as it apBoost Converter proaches full discharge. For these demanding applicaRSENSE The value of the sense resistor is the primary determin- tions, a FET with low VGS(th) is required. A good rule of ing factor for maximum output current. The SC1408 has thumb is that VGS(th) should be at least 0.5V less than the minimum input voltage. a fixed current limit voltage threshold, which is developed by the peak inductor current flowing through RSENSE. RSENSE may be determined either from the maxi- Diode mum output current curves or from the equation below: For most applications, a Schottky diode should be used as the output rectifier. It will be subjected to reverse voltages of at least VO , and average current will be equal to VCS ae VO + VF - VIN o c1 - / IO ( MAX ) = the output current. Industry standard 1N5817 series or RSENSE c VO + VF - VFET / e an equivalent surface mount part would be suitable. t off ae (VIN - VFET )(VO + VF - VIN ) o c - 2L c VO + VF - VFET e Output Capacitors Where : Output capacitors should be low ESR to minimize ripple voltage and maximize efficiency. Low ESR tantalums, VF = Output Diode Forward Voltage Drop OSCONs or the newer Polymer capacitors should be VFET = Voltage across FET, R SENSE and Inductor DCR used. Ripple voltage will be approximately: In the equation above, the use of 2.3s for toff may lead to slightly optimistic current values for low VO/VIN ratios. The theoretical curves use the actual value of toff, VF=0.5V, VFET=0.3V and VCS=0.08V and are generated for L=22H. Output Voltage Output voltage can be set to 5V by connecting the FB
VRIPPLE = VCS * R ESR R SENSE
pin to GND, or to any voltage in the 3.0V to 16.5V range using external divider resistors. The bottom resistor in the divider chain (R4) should be 300k or less and the top resistor (R3 in the application circuits) can be calculated from:
Input Capacitors Input capacitors on a boost converter are less critical than the output capacitors, since there are no fast current pulses drawn from the input supply. A 100F tantalum will be adequate for most applications. 7
(c) 2000 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
LOW VOLTAGE, BOOST DC-DC CONTROLLER
August 22, 2000 Feed Forward Capacitor Although converters based on the SC1408 are stable, with no possibility of oscillation, they are susceptable to "mode switching" at intermediate current levels. This leads to increased ripple voltage and slightly reduced efficiency. Mode switching can be eliminated by a suitable choice of C9. It is possible to empirically select C9. Start with a value of about 100pF and adjust until all low frequency output ripple is eliminated at the desired output current. See AN99-15 "SC1408 Operating Modes" for a full discussion of the origins of "Mode switching" and a more analytical approach to the selection of C9.
SC1408
Diode For most applications, a Schottky diode should be used as the output rectifier. It will be subjected to reverse voltages of at least VO +VIN and average current equal to the output current. Industry standard 1N5817 series or an equivalent surface mount part would be suitable. Output Capacitors Output capacitors should be low ESR to minimize ripple voltage and maximize efficiency. Low ESR tantalums, OSCONs or the newer Polymer capacitors should be used. Input Capacitors Input capacitors on a SEPIC converter are less critical than the output capacitors, since there are no fast current pulses drawn from the input supply. A 100F tantalum will be adequate for most applications. Series Capacitors The Series capacitor(s) must be capable of handling an RMS current given by:IRMS = IO VO + 0.5 VIN
COMPONENT SELECTION
SEPIC Converter RSENSE Again, with the SEPIC topology, the value of the sense resistor is the primary determining factor for maximum output current. The simplest approach to select RSENSE is to add Vin to Vo and use this value as the output voltage in the output current curves or in the equation for Boost converter. Output Voltage Output voltage setting works exactly the same in SEPIC topology as in Boost, including the ability to set to 5V by connecting the FB pin to GND. Care must be taken to ensure that the IC supply (pin2; BST) does not exceed its 16.5V rating. In the circuit of Fig.2: This requires maximum output voltage to be limited to 16.5V-Vin. Higher output voltages are possible with different IC supply strategies. Inductor The SEPIC topology requires a coupled inductor. Again a good choice for most applications is 22uH. Smaller inductor values result in higher peak currents and increase output ripple, while larger values will result in slower loop response. Transistor Selection The choice of FET can be critical, especially in battery powered applications where the converter must be able to use all of the available energy in the battery. This requires that the converter be capable of starting up from very low input voltages. For example a two cell alkaline system's terminal voltage will drop to 1.8V as it approaches full discharge. For these demanding applications, a FET with low VGS(th) is required. A good rule of thumb is that VGS(th) should be at least 0.5V less than the minimum input voltage.
Feed Forward Capacitor The feed forward capacitor should be selected in the same way as for a Boost application.
LAYOUT GUIDELINES
The SC1408 is fairly insensitive to layout, however following some simple guidelines will help ensure successful implementation. Whenever possible, lay the circuitry out over a ground plane. Connect AGND and GND together at the IC and return to the ground plane with one or two vias. Place the current sense resistor (R2), the REF decoupling capacitor (C5) and the divider resistors (R3 and R4) close to the IC, Return the ground side of these components to the ground plane close to the AGND/GND pin vias. Connect the top side of the sense resistor to ISENSE with a short trace and the source of Q1 directly to the sense resistor. Make the output loop Q1, D1 and the output capacitors (C3, C8) as small as possible to minimize EMI.
8 (c) 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
LOW VOLTAGE, BOOST DC-DC CONTROLLER
August 22, 2000
3.5 3.0 2.5 25mOhm Io (A)
Io (A) Vo=3.3V L=22uH 3.5 Vo=5V L=22uH
SC1408
20mOhm
3.0 2.5 2.0 1.5 1.0 0.5 0.0
20mOhm 25mOhm
2.0 1.5 1.0 0.5 0.0 2.0
3.5 3.0 2.5 Vo=12V L=22uH 20mOhm
35mOhm 50mOhm 100mOhm 2.2 2.4 2.6 Vin (V) 2.8 3.0 3.2
35mOhm 50mOhm 100mOhm
2.0 4.0 3.5 3.0
2.5
3.0
3.5 Vin (V)
4.0
4.5
5.0
Vo=15V L=22uH 20mOhm 25mOhm
25mOhm Io (A)
2.5 2.0 1.5
Io (A)
2.0 1.5 1.0 0.5 100mOhm 0.0 2.0 4.0 6.0 Vin (V) 8.0 10.0 12.0 35mOhm 50mOhm
35mOhm 50mOhm 100mOhm
1.0 0.5 0.0 2.0 4.0 6.0 8.0 10.0 Vin (V) 12.0
14.0
16.0
Figs. 5 - 8: Maximum output current vs. input voltage and sense resistor value. (Boost Mode)
100% Vo=5V 100% Vo=12V
90% Efficiency (%) Efficiency (%)
90%
80%
80%
70%
Vin=5.0V Vin=4.0V Vin=3.3V Vin=3.0V
70%
Vin=5.0V Vin=3.0V Vin=1.8V
60% 1 10 Io (mA) 100% Vo=15V 100 1000
60% 1 10 Io (mA) 100 1000
90% Efficiency (%)
80%
Vin=12V Vin=9.0V Vin=5.0V Vin=3.0V Vin=1.8V
70%
60% 1 10 Io (mA) 100 1000
Figs. 9-11:Efficiency in the Boost Application circuit of Fig.1
9 (c) 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
LOW VOLTAGE, BOOST DC-DC CONTROLLER
August 22, 2000
SC1408
Output Ripple Voltage; Vin=3V, Vo=5V, Io=470mA Ch1 = Output Ripple Ch2 = Voltage at GATE pin
Output Ripple Voltage; Vin=3V, Vo=5V, Io=810mA Ch1 = Output Ripple Ch2 = Voltage at GATE pin
Load Transient; Vin=3V, Vo=5V, Io=0 to 500mA Ch1 = Output Voltage Ch2 = Load Current (0.5A/div)
Load Transient; Vin=2V, Vo=5V, Io=0 to 500mA Ch1 = Output Voltage Ch2 = Load Current (0.5A/div)
10 (c) 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
LOW VOLTAGE, BOOST DC-DC CONTROLLER
August 22, 2000
SC1408
OUTLINE DRAWING - SO-8
JEDEC REF: MS-012AA
LAND PATTERN - SO-8
11 (c) 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
LOW VOLTAGE, BOOST DC-DC CONTROLLER
August 22, 2000
SC1408
OUTLINE DRAWING - MSOP-8
LAND PATTERN - MSOP-8
ECN00-1275
12 (c) 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320

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