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LTC1703

FCB (Pin 7): Force Continuous Bar. The FCB pin forces

both converters to maintain continuous synchronous

operation regardless of load when the voltage at FCB

drops below 0.8V. FCB is normally tied to VCC. To force

continuous operation, tie FCB to SGND. FCB can also be

connected to a feedback resistor divider from a secondary

winding on one converter’s inductor to generate a third

regulated output voltage. Do not leave FCB floating.

RUN/SS1 (Pin 8): Controller 1 Run/Soft-Start. Pulling

RUN/SS1 to SGND will disable controller 1 and turn off

both of its external MOSFET switches. Pulling both

RUN/SS pins down will shut down the entire LTC1703,

dropping the quiescent supply current below 100

µA. A

capacitor from RUN/SS1 to SGND will control the turn-on

time and rate of rise of the controller 1 output voltage at

power-up. An internal 3.5

µA current source pull-up at

RUN/SS1 pin sets the turn-on time at approximately

50ms/

µF.

COMP1 (Pin 9): Controller 1 Loop Compensation. The

COMP1 pin is connected directly to the output of the first

controller’s error amplifier and the input to the PWM

comparator. An RC network is used at the COMP1 pin to

compensate the feedback loop for optimum transient

response.

SGND (Pin 10): Signal Ground. All internal low power

circuitry returns to the SGND pin. Connect to a low

impedance ground, separated from the PGND node. All

feedback, compensation and soft-start connections should

return to SGND. SGND and PGND should connect only at

a single point, near the PGND pin and the negative plate of

the CIN bypass capacitor.

FB1 (Pin 11): Controller 1 Feedback Input. The loop

compensation network for controller 1 should be con-

nected to FB1. FB1 is connected internally to the VID

resistor network to set the output voltage at side 1.

SENSE (Pin 12): Output Sense. Connect to VOUT1.

VID0 to VID4 (Pins 13 to 17): VID Programming Inputs.

These are logic inputs that set the output voltage at side 1

to a preprogrammed value (see Table 1). VID4 is the MSB,

VID0 is the LSB. The codes selected by the VID

n inputs

correspond to the Intel Mobile VID specification. Each

PVCC (Pin 1): Driver Power Supply Input. PVCC provides

power to the two BG

n output drivers. PVCC must be

connected to a voltage high enough to fully turn on the

external MOSFETs QB1 and QB2. PVCC should generally

be connected directly to VIN. PVCC requires at least a 1µF

bypass capacitor directly to PGND.

BOOST1 (Pin 2): Controller 1 Top Gate Driver Supply. The

BOOST1 pin supplies power to the floating TG1 driver.

BOOST1 should be bypassed to SW1 with a 1

µF capacitor.

An additional Schottky diode from VIN to BOOST1 pin will

create a complete floating charge-pumped supply at

BOOST1. No other external supplies are required.

BG1 (Pin 3): Controller 1 Bottom Gate Drive. The BG1 pin

drives the gate of the bottom N-channel synchronous

switch MOSFET, QB1. BG1 is designed to drive up to

10,000pF of gate capacitance directly. If RUN/SS1 goes

low, BG1 will go low, turning off QB1. If FAULT mode is

tripped, BG1 will go high and stay high, keeping QB1 on

until the power is cycled.

TG1 (Pin 4): Controller 1 Top Gate Drive. The TG1 pin

drives the gate of the top N-channel MOSFET, QT1. The

TG1 driver draws power from the BOOST1 pin and returns

to the SW1 pin, providing true floating drive to QT1. TG1

is designed to drive up to 10,000pF of gate capacitance

directly. In shutdown or fault modes, TG1 will go low.

SW1 (Pin 5): Controller 1 Switching Node. SW1 should be

connected to the switching node of converter 1. The TG1

driver ground returns to SW1, providing floating gate

drive to the top N-channel MOSFET switch, QT1. The

voltage at SW1 is compared to IMAX1 by the current limit

comparator while the bottom MOSFET, QB1, is on.

IMAX1 (Pin 6): Controller 1 Current Limit Set. The IMAX1

pin sets the current limit comparator threshold for

controller 1. If the voltage drop across the bottom MOSFET,

QB1, exceeds the magnitude of the voltage at IMAX1,

controller 1 will go into current limit. The IMAX1 pin has an

internal 10

µA current source pull-up, allowing the current

threshold to be set with a single external resistor to PGND.

This current setting resistor should be Kelvin connected to

the source of QB1. See the Current Limit Programming

section for more information on choosing RIMAX.

PI FU CTIO S

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