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SC2604
POWER MANAGEMENT Features

Simple PWM Boost Controller with Input Disconnect FET Drive
Description
The SC2604 is a versatile, low-cost, voltage-mode PWM controller designed for boost DC/DC power supply applications. It features input disconnect FET driver allowing power source and load separation at shutdown mode, which eliminates possible leakage current from source to load. Also, it prevents catastrophic failure when output is shorted during operation. The SC2604 also includes temperature compensated voltage reference, internal ramp, current limit comparator, internally compensated error amplifier, and floating driver with charge pump. Programmable soft start controls in-rush current and reduces output voltage overshoot. Hiccup mode over-current protection allows system autoretry and ease of trouble shooting. Internally compensated feedback loop makes power supply design simple, and eliminates the need for external compensation network. The SC2604 is available in MSOP-8 package with rated temperature range of -40oC to +85oC.
Input Voltage Range: 4.5V to 3.5V % Voltage Reference Accuracy Up to 95% Efficiency Input Disconnect FET Drive In-rush Current Control Internal Compensation Programmable Current Limit Programmable Soft Start 800mA Typical PWM Gate Drive 400kHz Switching Frequency Under Voltage Lockout <200uA Shutdown Current -40oC to +85oC Temperature Range MSOP-8 Package, Fully WEEE and RoHS Compliant
Applications

Portable Devices Flat Panel TV TV Set Top Box Auxiliary Supplies Peripheral Card Supplies Industrial Power Supply High Density DC/DC Conversion
Typical Application Circuit
Figure 1. 12V to 25V/1A Boost Converter with Over Current Protection
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SC2604
Pin Configuration Ordering Information
Device
SC2604MSTRT()(2) SC2604EVB
Package
MSOP-8 Evaluation Board
Notes: () Available in tape and reel only. A reel contains 2,500 devices. (2) Available in lead-free package only. Device is fully WEEE and RoHS compliant. (8 - Pin MSOP)
Marking Information
Top Mark
Bottom Mark
nnnn=Part Number Code (Example AS00) - Reference Part No. Code for MSOP yyww=Date Code (Example: 0752) xxxx = Semtech Lot No. (Example: E90) xxxx = Semtech Lot No. (Example: 0-)
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SC2604
Absolute Maximum Ratings
VIN Supply Voltage .................................... -0.3 to 20V CS Pin Voltage.............................................-0.3 to 20V GATE Pin Voltage..........................................-0.3 to 20V DRV Pin Voltage ..........................................-0.3 to 25V OCP/EN Pin Voltage .......................................-0.3 to 7V SS/VREF Pin Voltage .......................................-0.3 to 7V FB Pin Voltage .............................................-0.3 to 7V Peak IR Reflow Temperature ............................... 260C ESD Protection Level(2) ....................................... 2000V
Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not recommended. NOTES() Calculated from package in still air, mounted to 3" x 4.5", 4 layer FR4 PCB with thermal vias under the exposed pad per JESD5 standards. (2) Tested according to JEDEC standard JESD22-A4-B.
Thermal Information
Junction to Ambient () .................................... 60C/W Junction to Case () ....................................... 45C/W Maximum Junction Temperature........................... 50C Storage Temperature .............................. -45 to +50C Lead Temperature (Soldering) 0 sec ..................... 300C
Recommended Operating Conditions
Input Voltage Range ................................. 4.5V to 3.5V
Electrical Characteristics
Unless otherwise noted, VIN = 2V, VO = 25V, -40C < TA = TJ < 25C.
Parameter Input Supply
VIN Supply Voltage VIN Start Voltage VIN Start Hysteresis VIN Supply Current VIN Shutdown Current
Conditions
Min
4.5
Typ
Max
3.5
Units
V V mV
VIN Rising
4.2 400
4.5
Switching, GATE pin floating OCP/EN = Low
6.0
9.0 200
mA A
Error Amplifier
Feedback Voltage Feedback Bias Current Error Amplifier Gain
()
IO = 00mA VIN = 2V, VFB = VSS/VREF
.225
.250 0.5 90
.275 .0
V
A
V/V
480 kHz % V V
Oscillator
Oscillator Frequency Maximum Duty Cycle Internal Ramp Peak (2) Internal Ramp Valley (2) 320 86 400 90 .4 0.4
Regulation
Load Regulation Line Regulation IO = 0.A to A VIN = 5V to 3.5V, IO = 0.A 0.5 .0 % %
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SC2604
Electrical Characteristics (Cont.)
Unless otherwise noted, VIN = 2V, VO = 25V, -40C < TA = TJ < 25C.
Parameter PWM Switch Gate Drive
Gate Source Current Gate Sink Current
Conditions
VIN = 2V, CGATE = 0nF VIN = 2V, CGATE = 0nF
Min
0.5 0.5
Typ
0.8 0.8
Max
Units
A A
PWM Switch Soft Start
Soft Start Charge Current SS/VREF Threshold to Shutdown Switch Pull down below this level to disable PWM Switch gate Pull above this level to enable PWM Switch gate 30 55 00 A mV
SS/VREF Threshold to Turn-on Switch
mV
Disconnect Switch Gate Drive
DRV Source Current DRV Sink Current VIN = 2V, VDRV = 5.5V VIN = 2V, VDRV = 8V 45 45 A A
Over Current Protection
Current Limit Threshold OCP/EN Threshold OCP/EN Charge Current OCP/EN Discharge Current CS Input Current Note: (). Guaranteed by Characterization (2). Guaranteed by design VIN - CS Pull down below this level to disable Disconnect FET gate 6 520 72 590 37 .0 0.2 83 660 mV mV A A A
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SC2604
Pin Descriptions
Pin
2 3 4 5 6
Pin Name
CS VIN GATE GND SS/VREF FB
Pin Function
Current sense input (negative) Device supply voltage (also positive current sense input) PWM gate driver output for boost converter Device ground Soft start and reference voltage pin Error amplifier inverted input When a capacitor is tied to this pin, the maximum inrush current is controlled during start-up. The capacitor value also determines the off-time after the device has entered hiccup mode. Pulling this pin low can disable the linear and the switcher to turn off the circuit. Gate drive of input disconnect FET limiting system input current
7
OCP/EN
8
DRV
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SC2604
Block Diagram
Figure 2. SC2604 Function Diagram
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SC2604
Typical Characteristics
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SC2604
Typical Characteristics (Cont.)
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SC2604
Applications Information
PWM Control Loop The SC2604 is a voltage-mode PWM controller with a fixed switching frequency of 400kHz for use in high efficiency, boosted voltage, DC/DC power supplies. As shown in Figure 2, the PWM control loop of the SC2604 consists of a 400kHz oscillator, a PWM comparator, a voltage error amplifier, and a FET driver. The boost converter output voltage is fed back to FB (error amplifier negative) and is regulated to the reference voltage at SS/VREF pin. The error amplifier output is compared with the 400kHz ramp to generate a PWM wave, which is amplified and used to drive the boost FET (Q2 in Figure ) for the converter. The PWM controller works with soft start and fault monitoring circuitry to meet application requirements. UVLO, Start-up, and Shutdown To initiate the SC2604, a supply voltage is applied to VIN. The DRV and GATE are held low. When VIN voltage exceeds UVLO (Under Voltage Lockout) threshold, typically 4.2V, an internal current source (37A) begins to charge the OCP/EN pin capacitor. The OCP/EN voltage ramps from near ground to over .25V but the voltage between 0.625V and .25V provides the linear soft-start range for the disconnect FET (Q). When the OCP/EN voltage is over .25V, the OCP hiccup is enabled, and SS/VREF pin is released. At this moment, another internal current source (55A) begins to charge the SS/VREF pin capacitor. When the SS/VREF pin voltage reaches 0.5V, the error amplifier output will rise to 0.4V, then the PWM comparator begins to switch. The switching regulator output is slowly ramping up for a soft turn-on. The details of SC2604 startup timing is shown in Figure 3. If the supply voltage at VIN pin falls below UVLO threshold (3.8V typically) during a normal operation, the DRV pin is pulled low to cut off the supply power of the boost converter, while the OCP/EN pin capacitor is discharged with a A internal current source. When the OCP/EN pin falls below .25V, the SS/VREF pin is forced to ground. This completely shuts down the boost conveter. Directly pulling the OCP/EN pin below 0.52V can also (c) 2007 Semtech Corp. allow a complete shutdown of the output. Pulling the SS/ VREF pin below 0.V only shuts the boost FET (Q2 in Figure ) off and the output voltage will be (VIN-Vd).
Figure 3. Start-up Timing Diagram
Hiccup Mode Short Circuit Protection Hiccup mode over-current protection is utilized in the SC2604. When an increasing load causes a voltage of 72mv to occur from VIN to CS then a current limit hiccup sequence is started. The sequence starts by pulling DRV low and discharging the OCP/EN pin with a A current source. When the OCP/EN pin falls below .25V, the SS/ VREF pin is forced to ground (similar to the UVLO shutdown described in the last setion). When the voltage on the OCP/EN pin falls to near zero volt, the A discharge current becomes a 37A charging current and the OCP/EN pin starts to charge and DRV is enabled. When the OCP/EN voltage rises from 0.625V to .25V, the current in the disconnect FET is allowed to increase from zero to a maximum of 72mV/(Current Sense Resistor Value). If the over-current condition still exists when OCP/EN crosses .25V then the hiccup sequence will re-start. If there is no over-current as OCP/EN crosses .25V then the SS/VREF pin is released to rise and allow a
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SC2604
Applications Information (Cont.)
soft-start of the switching boost regulator. The DRV pin of the SC2604 is meant to drive an N-Channel FET that can disconnect the input supply in the event of an over-current condition. The OCP/EN capacitor becomes part of a hiccup oscillator that is charged with 37A and discharged with A to provide a low duty cycle for the FET Q. It should be understood that sufficiently fast ramp rates on the OCP/EN pin and the SS/VREF pin can trigger a hiccup event because of the charging current demanded by the boost regulator output capacitor. Disconnect FET Selection Setting the Output Voltage In Figure , an external resistive divider R7 and R8 with its center tap tied to the FB pin sets the output voltage. The floating driving voltage of DRV pin drops slightly as the supply voltage VIN is below 7.5V (Typical Characteristics on page 8), where a FET with low gate threshold voltage (VGS(TH)) has to be used for the disconnect FET. In a 5V input application, a FET with VGS(TH)=2V, such as FDD6672A from Fairchild, is needed. Layout Guidelines Careful attentions to layout requirements are necessary for successful implementation of the SC2604 PWM controller. High currents switching at 400kHz are present in the application and their effect on ground plane voltage differentials must be understood and minimized. ) The high power parts of the circuit should be laid out first. A ground plane should be used, the number and position of ground plane interruptions should be such as to not unnecessarily compromise ground plane integrity. Isolated or semi-isolated areas of the ground plane may be deliberately introduced to constrain ground currents to particular areas, for example the input capacitor and bottom Schottky ground. 2) The loop formed by the output Capacitor(s) (COUT ), the FET (Q), the current sensing resistor, and the Schottky (D) must be kept as small as possible, as shown on the layout diagram in Figure 4. This loop contains all the high current, fast transition switching. Connections should be as wide and as short as possible to minimize loop inductance. Minimizing this loop area will reduce EMI,
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Capacitor at OCP/EN Pin - COCP/EN As the current at start-up may hit its current limit threshold, the ramp rate of the current must be slow enough to allow the output capacitor to be fully charged to a voltage one diode drop Vd less than input voltage VIN. To guarantee a successful start-up at no load, the value of the capacitor at the OCP/EN pin has to satisfy the following formula:
In some applications, a RC branch (R6, C2 in the Typical Schematic on page 2) will be needed for loop stability. Maximum Duty Cycle The maximum duty cycle, Dmax defines the upper limit of power conversion ratio
Calculating Current Sense Resistor Current sense resistor is placed at the input to sense inductor peak current of the boost regulator. The value of the resistor can be calculated by
where IPEAK is the allowed boost inductor peak current. In many applications, a noise filter circuit (R=200, C0=0nF in the Typical Schematic on page 2) may be needed for the input current sensing. (c) 2007 Semtech Corp.
SC2604
Applications Information (Cont.)
lower ground injection currents, resulting in electrically "cleaner" grounds for the rest of the system and minimize source ringing, resulting in more reliable gate switching signals. 3) The connection between the junction of Q, D and the output capacitor should be a wide trace or copper region. It should be as short as practical. Since this connection has fast voltage transitions, keeping this connection short will minimize EMI. 4) The Output Capacitor(s) (COUT ) should be located as close to the load as possible, fast transient load currents are supplied by COUT only, and connections between COUT and the load must be short, wide copper areas to minimize inductance and resistance. 5) The SC2604 is best placed over an isolated ground plane area. The soft-start capacitor and the Vin decoupling capacitor should also connected to this ground pad area. This isolated ground area should be connected to the main ground by a trace that runs from the GND pin to the ground side of the output capacitor. If this is not possible, the GND pin may be connected to the ground path between the Output Capacitor and the CIN, Q, D loop. Under no circumstances should GND be returned to a ground inside the CIN, Q, D loop. 6) Input voltage of the SC2604 should be supplied from the power rail through a resistor, the Vin pin should be decoupled directly to GND by a 0.F~F ceramic capacitor, trace lengths should be as short as possible.
Note: Heavy lines indicate the critical loop carrying high pulsating current. The inductance of the loop needs to be minimized.
Figure 4. SC2604 Layout Diagram (c) 2007 Semtech Corp.
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SC2604
Applications Information (Cont.)
Typical application schematic with 12V input and 25V/1.5A output
Note: A small Schottky diode (Da) may be required in some applications to clamp negative spike at the GATE pin.
Bill of materials
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SC2604
Applications Information (Cont.)
Start up
Inductor current and DRV pin voltage at OCP
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SC2604
Outline Drawing - MSOP-8
Land Pattern - MSOP-8
Contact Information
Semtech Corporation Power Mangement Products Division 200 Flynn Road, Camarillo, CA 9302 Phone: (805) 498-2 Fax: (805) 498-3804
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