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| HIGH-EFFICIENCY STEP-DOWN DC-DC CONVERTER January 29, 1998 SC1578 TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com DESCRIPTION The SC1578 is a high performance step-down DC-DC converter, designed to drive an external P-channel MOSFET to generate programmable output voltages. Two main schemes of pulse-skipping and pulsefrequency modulation are employed to maintain low quiescent current and high conversion efficiency over wide ranges of input voltage and loads. The SC1578 delivers 10mA to 2A of output current with 87%-93% efficiency at VIN=9V, VOUT=5V. A current sense comparator with both inverting and non-inverting inputs uncommitted is included to provide the crucial function of either current limit protection or constant output current control. When the SC1578 is used in a highside current sensing step-down constant current source, the efficiency is typically greater than 90%. Duty cycle can be adjusted to greater than 90% by connecting a resistor from the DUTY pin to VIN. Quiescent current is about 90A and can be reduced to 8A in shutdown mode. With a switching frequency range of 90kHz to 280kHz, small size switching components may be used, which is ideal for battery powered portable equipment. FEATURES * 4V to 24V input voltage operation. * High efficiency (up to 95%). * Low quiescent current at 90A. * Pulse-skipping and pulse-frequency modulation. * Inputs-uncommitted current sense comparator. * Duty cycle adjustable * 90kHz to 280kHz oscillator frequency * Power-saving shutdown mode (8A Typical). * Push-pull driver output APPLICATIONS * Notebook 5V/3.3V main power * Step-down DC-DC converter module. * Constant current source for battery chargers. BLOCK DIAGRAM ORDERING INFORMATION DEVICE (1) PACKAGE SO-8 SC1578CS Note: (1) Add suffix `TR' for tape and reel. PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS Parameter Supply Voltage Duty Voltage SHDN Voltage Operating Temperature Range Storage Temperature Range Symbol VIN VDUTY VSHDN TA TSTG Maximum 24 20 15 0 to 70 -65 to 150 Units V V V C C (c) 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 HIGH-EFFICIENCY STEP-DOWN DC-DC CONVERTER January 29, 1998 SC1578 ELECTRICAL CHARACTERISTICS Unless otherwise specified, TA = 25C, VIN = 13V Parameter Operation Voltage Quiescent Current Shutdown Mode Current Internal Reference Voltage Driver Sinking "On resistance" Driver Sourcing "On resistance" Current Sense Comparator Threshold Shutdown Threshold SHDN Pin Leakage Current Duty Cycle Oscillator Frequency VSHDN < 15V VDUTY = VIN VDUTY = VIN 71 225 1578 1578A VCS+ = 13V 42 52 0.8 VFB = 1.5V VSHDN = 0V 1.16 Conditions Min 4 90 8 1.22 16 11 57 57 1.5 72 62 2.4 1 Typ Max 24 160 20 1.28 Units V A A V mV V A % kHz TYPICAL PERFORMANCE CHARACTERISTICS (c) 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 HIGH-EFFICIENCY STEP-DOWN DC-DC CONVERTER January 29, 1998 SC1578 TYPICAL APPLICATION CIRCUIT APPLICATION EXAMPLES to , Figure 1. 5V Step-Down Converter to Figure 2. 3.3 Step-Down Converter (c) 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 HIGH-EFFICIENCY STEP-DOWN DC-DC CONVERTER January 29, 1998 SC1578 TYPICAL APPLICATION CIRCUIT Figure 3. Battery Charge Circuit with High-Side Current Sensing Constant Current Source NOTE: RSENSE=0.1 ohm, charge current=0.5A 10%, VIN>VBATT + 3.5V RSENSE=0.05 ohm, charge current=1A 10%, VIN>VBATT + 4V RSENSE=0.033 ohm, charge current=1.5A 10%, VIN>VBATT + 4.5V Efficiency>90%, measured at CS- node PIN DESCRIPTIONS PIN 1: VIN 4V to 24V input supply voltage. PIN 5: GND Power ground. PIN 6: DRI Push-Pull driver output to drive an external P-channel MOSFET or PNP transistor. When driving a PNP bipolar transistor, a base resistor and a capacitor to the base of PNP are recommended. Current sense comparator inverting input, not to exceed VIN. PIN 2: DUTY Duty cycle adjustment pin. To be tied to the VIN pin directly or through a resistor to adjust oscillator duty cycle. See Electrical Characteristics: RDUTY must be above 1M if VIN>20V. PIN 3: SHDN Logical input to shutdown the chip: VSHDN = high for normal operation, VSHDN = low for shutdown. This pin should not be left floating or forced greater than 15V. In shutdown mode the DRI pin is high. PIN 4: FB Feedback comparator input, to compare the feedback voltage with the internal reference voltage. Connecting a resistor R1 to the converter output and a resistor R2 to ground yields the output voltage: VOUT = 1.22 x (R1+R2)/R2 PIN 7: CS- PIN 8: CS+ Current sense comparator noninverting input, not to exceed VIN. (c) 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 HIGH-EFFICIENCY STEP-DOWN DC-DC CONVERTER January 29, 1998 SC1578 DEVICE OUTLINE - SO-8 LAND PATTERN - SO-8 (c) 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 |
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