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