Datasheet LT1943 (Analog Devices) - 7

LT1943
U U U PI FU CTIO S GND (Pins 1, 20, Exposed Pad Pin 29):
Ground. Tie both
VC4 (Pin 10):
Switching Regulator 4 Error Amplifier GND pins and the exposed pad directly to a local ground Compensation. Connect a resistor/capacitor network in plane. The ground metal to the exposed pad should be as series with this pin. wide as possible for better heat dissipation. Multiple vias
SGND (Pin 11):
Signal Ground. Return ground trace from (to ground plane under the ground backplane) placed the FB resistor networks and V close to the exposed pad can further aid in reducing C pin compensation compo- nents directly to this pin and then tie to ground. thermal resistance.
BOOST (Pin 12):
The BOOST pin is used to provide a drive
VC1 (Pin 2):
Switching Regulator 1 Error Amplifier Com- voltage, higher than V pensation. Connect a resistor/capacitor network in series IN, to the switch 1 drive circuit. with this pin.
SW1 (Pins 13, 14):
The SW1 pins are the emitter of the internal NPN bipolar power transistor for switching regu-
VC2 (Pin 3):
Error Amplifier Compensation for Switcher 2. lator 1. These pins must be tied together for proper Connect a resistor/capacitor network in series with this operation. Connect these pins to the inductor, catch diode pin. and boost capacitor.
FB1 (Pin 4):
Switching Regulator 1 Feedback. Tie the
V
resistor divider tap to this pin and set V
IN (Pins 15, 16):
The VIN pins supply current to the LOGIC according to LT1943’s internal regulator and to the internal power VLOGIC = 1.25 • (1 + R2/R1). Reference designators refer transistor for switch 1. These pins must be tied together to Figure 1. and locally bypassed.
FB2 (Pin 5):
Feedback for Switch 2. Tie the resistor divider
SS-234 (Pin 17):
This is the soft-start pin for switching tap to this pin and set AVDD according to AVDD = 1.25 • regulators 2, 3 and 4. Place a soft-start capacitor here to (1 + R6/R5). limit start-up inrush current and output voltage ramp rate.
FB3 (Pin 6):
Switching Regulator 3 Feedback. Tie the When the BIAS pin reaches 2.8V, a 1.7µA current source resistor divider tap to this pin and set VON according to begins charging the capacitor. When the capacitor voltage VON = 1.25 • (1 + R9/R8) – 150mV. reaches 0.8V, switches 2, 3 and 4 turn on and begin
NFB4 (Pin 7):
Switching Regulator 4 Negative Feedback. switching. For slower start-up, use a larger capacitor. Switcher 4 can be used to generate a positive or negative When this pin is pulled to ground, switches 2, 3 and 4 are disabled. For complete shutdown, tie RUN-SS to ground. output. When regulating a negative output, tie the resistor divider tap to this pin. Negative output voltage can be set
RUN-SS (Pin 18):
This is the soft-start pin for switching by the equation VOFF = –1.245 • (R3/R4) with R4 set to 10k. regulator 1. Place a soft-start capacitor here to limit start- Tie the NFB4 pin to FB4 for positive output voltages. up inrush current and output voltage ramp rate. When
FB4 (Pin 8):
Feedback for Switch 4. When generating a power is applied to the VIN pin, a 1.7µA current source positive voltage from switch 4, tie the resistor divider tap charges the capacitor. When the voltage at this pin reaches 0.8V, switch 1 turns on and begins switching. For slower to this pin. When generating a negative voltage, tie a 10k resistor between FB4 and NFB4 (R4). start-up, use a larger capacitor. For complete shutdown, tie RUN-SS to ground.
VC3 (Pin 9):
Switching Regulator 3 Error Amplifier Com-
SW4 (Pin 19):
This is the collector of the internal NPN pensation. Connect a resistor/capacitor network in series with this pin. bipolar power transistor for switching regulator 4. Mini- mize metal trace area at this pin to keep EMI down. 1943fa 7