SC1650

HIGH-VOLTAGE INVERTING DC-DC CONVERTER January 29, 1998 SC1650 TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com DESCRIPTION The SC1650 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, maximum efficiency of 90% can be achieved with low cost PNP bipolar transistor drivers. The 4V to 24V input operation range allows the SC1650 to be powered directly by the battery pack in most batteryoperated applications for greater efficiency. 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 100µA and can be reduced to 8µA in shutdown mode. With a switching frequency range of 100kHz to 320kHz, small size switching components may be used, which is ideal for battery powered portable equipment such as notebook and palmtop computers. FEATURES • 4V to 24V input voltage operation • Adjustable output voltage up to -40V • Low quiescent current at 100µA • Pulse frequency modulation maintains high • • • efficiency (max 90%) 100kHz to 320kHz switching frequency Power-Saving shutdown mode (8µA 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 SC1650CS Note: (1) Add suffix ‘TR’ for tape and reel. ABSOLUTE MAXIMUM RATINGS Parameter Supply Voltage Symbol V+ VSHDN TA Maximum 24 15 0 to 70 Units V V °C PIN CONFIGURATION SHDN Voltage Operating Temperature Range Storage Temperature Range TSTG -65 to 125 °C © 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 HIGH-VOLTAGE INVERTING DC-DC CONVERTER January 29, 1998 SC1650 ELECTRICAL CHARACTERISTICS Unless otherwise specified, TA = 25°C, V+ = 13V 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 15 10 60 1.5 75 2.4 VFB = -50mV Min 4 100 8 1.22 Test Limits Typ Max 24 200 20 1.28 Units V µA µA V µA Ω Ω mV V TYPICAL PERFORMANCE CHARACTERISTICS TYPICAL APPLICATION CIRCUIT © 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 HIGH-VOLTAGE INVERTING DC-DC CONVERTER January 29, 1998 SC1650 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 determing the optimal component val- L = 11 .1− 0 .15 × V + × + ( ) IOUT V+ × VOUT PNP : VCEO > V + VOUT IC ( MAX ) ≥ 200 × IOUT V+ 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 HIGH-VOLTAGE INVERTING DC-DC CONVERTER January 29, 1998 SC1650 OUTLINE DRAWING - SO-8 PIN CONFIGURATION PIN DESCRIPTIONS PIN 1: V + 4V to 24V 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 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: V OUT = R1 * V REF R2 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 PIN 5: GND Power ground. © 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320