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 (1) DEVICE VOUT PACKAGE SC1650CS Adj SO-8 Note: (1) Add suffix ‘TR’ for tape and reel. ABSOLUTE MAXIMUM RATINGS Parameter PIN CONFIGURATION Symbol Maximum Units Supply Voltage V+ 24 V SHDN Voltage VSHDN 15 V TA 0 to 70 °C TSTG -65 to 125 °C Operating Temperature Range Storage Temperature Range © 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 HIGH-VOLTAGE INVERTING DC-DC CONVERTER SC1650 January 29, 1998 ELECTRICAL CHARACTERISTICS Unless otherwise specified, TA = 25°C, V+ = 13V Test Conditions Parameter Test Limits Min Input Voltage Max Units 24 V 100 200 µA 8 20 µA 1.22 1.28 V 4 Switch Off Current VFB = -50mV Shutdown Mode Current VREF Voltage Typ ISOURCE=250µA VREF Source Current 1.16 250 µA DLOW “ON Resistance” 15 Ω DHI “ON Resisance” 10 Ω CL Threshold 45 60 75 mV Shutdown Threshold 0.8 1.5 2.4 V TYPICAL PERFORMANCE CHARACTERISTICS TYPICAL APPLICATION CIRCUIT © 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320 HIGH-VOLTAGE INVERTING DC-DC CONVERTER SC1650 January 29, 1998 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 SC1650 January 29, 1998 OUTLINE DRAWING - SO-8 PIN CONFIGURATION PIN DESCRIPTIONS + PIN 1: V 4V to 24V input supply voltage. PIN 2: VREF 1.22V reference output. Bypass with a 0.047µF capacitor to GND. Sourcing capability is guaranteed to be greater than 250µA. 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 = - PIN 5: GND 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. PIN 8: CL Current-limit input. This pin clamps the switch peak current under abnormal conditions. Power ground. © 1997 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320