SC1652

MICROPOWER INVERTING DC-DC CONVERTER January 29, 1998 SC1652 TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com DESCRIPTION The SC1652 is a high performance inverting DC-DC converter, designed to drive an external power switch to generate programmable negative voltages. In the particularly suitable LCD bias contrast application, efficiency of 87% can be achieved with low cost PNP bipolar transistor drivers. The output voltage can be scaled to -40V by two external resistors. A pulse frequency modulation scheme is employed to maintain high efficiency conversion under wide input voltage ranges. Quiescent current is about 80µA and can be reduced to 0.7µA in shutdown mode. With a switching frequency range of 70kHz to 160kHz, small size switching components may be used, which is ideal for battery powered portable equipment such as notebook and palmtop computers. FEATURES • 2.4V to 7V input voltage operation • Adjustable output voltage up to -40V • Low quiescent current at 80µA • Pulse frequency modulation maintains high • • • efficiency (max 87%) 70kHz to 160kHz switching frequency Power-saving shutdown mode (0.7µA typical) High efficiency with low cost external PNP bipolar transistor APPLICATIONS • Negative LCD Contrast Bias for 1. Notebook & palmtop computers 2. Pen-based data systems 3. Portable data collection terminals 4. Personal digital assistants Negative voltage supplies BLOCK DIAGRAM • ORDERING INFORMATION DEVICE (1) VOUT Adj PACKAGE SO-8 SC1652CS Note: (1) Add suffix ‘TR’ for tape and reel. PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS Parameter Supply Voltage Operating Temperature Range Storage Temperature Range Symbol V+ TA TSTG Maximum 7.0 0 to 70 -65 to 125 Units V °C °C © 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 MICROPOWER INVERTING DC-DC CONVERTER January 29, 1998 SC1652 ELECTRICAL CHARACTERISTICS Unless otherwise specified, TA = 25°C, V+ = 5V Test Conditions Parameter Input Voltage Switch Off Current Shutdown Mode Current VREF Voltage VREF Source Current DLOW “ON Resistance” DHI “ON Resisance” CL Threshold Shutdown Threshold 45 0.8 ISOURCE=250µA 1.16 250 5 7 60 1.5 75 2.4 Min 2.4 80 0.7 1.22 Test Limits Typ Max 7 150 2 1.28 Units V µA µA V µA Ω Ω mV V TYPICAL PERFORMANCE CHARACTERISTICS TYPICAL APPLICATION CIRCUIT . to to . © 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 MICROPOWER INVERTING DC-DC CONVERTER January 29, 1998 SC1652 TYPICAL APPLICATION INFORMATION The typical application circuit generates an adjustable negative voltage for contrast bias of LCD displays. Efficiency and output power can be optimized by using the appropriate inductor and switch. The following formulas provide a guideline for determining the optimal component values: L = 11 .1− 0.15 × V + × ( ) V+ IOUT × VOUT IOUT V+ PNP : VCEO > V + + VOUT IC(MAX ) ≥ 200 × IOUT and β = 10 V+ R B ≅ 3 × L × V + − 0.8 , where units : V + in Volt, VOUT in Volt, VCE ( SAT ) < 0.4 V at IC = 200 × ( ) IOUT in Ampere , L in µH, R B in Ohm. © 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 MICROPOWER INVERTING DC-DC CONVERTER January 29, 1998 SC1652 OUTLINE DRAWING SO-8 PIN CONFIGURATION PIN DESCRIPTIONS PIN 1: V + 2.4V to 7V input supply voltage. 1.22V reference output. Bypass with a 0.047µF capacitor to GND. Sourcing capability is guaranteed to be greater than 250µA. PIN 5: GND Power ground. PIN 2: VREF PIN 3: SHDN Logic input to shutdown the chip. >1.5V = normal operation, GND = shutdown. In shutdown mode DLOW and DHI pins are high. PIN 4: FB Feedback signal input to comparator. Connecting a resistance R1 to VOUT and a resistance R2 to VREF yields the output voltage: PIN 6: DLOW Driver sinking output. Connected to DHI when using an external P-channel MOSFET. When using an external PNP bipolar transistor, connect a resistor RB from this pin to DHI. RB value depends upon V+, inductor value and the PNP bipolar transistor. By adjusting the RB value, efficiency can be optimized. PIN 7: DHI Driver sourcing output. Connect to the gate of the external P-channel MOSFET or base of the PNP bipolar transistor. Current-limit input. This pin clamps the switch peak current under abnormal conditions. PIN 8: CL VOUT R1 =× VREF R2 © 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320