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LT3992EFE

Monolithic Dual Tracking 3A Step-Down Switching Regulator

Linear Technology 

LT3992

Pin Functions

BST1/2: The BST pin provides a higher than VIN base

drive to the power NPN to ensure a low switch drop. If the

voltage between the BST pin and the VIN pin is less than

the voltage required to fully turn on the power NPN, the

power switch is turned off to recharge the BST capacitor.

CMPI1/2: The CMPI pin is an input to a comparator with a

threshold of 725mV and 80mV of hysteresis. Connecting

the CMPI pin to the FB pin will generate a power good

signal when the output is within 90% of its regulated value.

CMPO1/2: The CMPO pin is an open-collector output that

sinks current when the CMPI pin falls below its threshold.

For a typical input voltage above 2.9V, its output state re-

mains true, although during shutdown, VIN1 undervoltage

lockout or thermal shutdown, its current sink capability is

reduced. The COMPO pins can be left open circuit or tied

together to form a single power good signal.

DIV: The voltage present at the DIV pin determines the ratio

of channel 1 frequency to the master clock frequency set

by the RT/SYNC pin. The DIV pin is driven by an internal

current source with a typical value of 12µA which allows

a single resistor from the DIV pin to ground to set the

DIV voltage and resulting channel 1 frequency divider.

Ratios of 1, 2, 4 and 8 are available. See the Applications

Information section for more information.

DNC: Do Not Connect.

GND: The exposed pad pin is the only ground connec-

tion for the device. The exposed pad should be soldered

to a large copper area to reduce thermal resistance. The

GND pin is common to both channels and also serves as

small-signal ground. For ideal operation all small-signal

ground paths should connect to the GND pin at a single

point avoiding any high current ground returns.

FB1/2: The FB pin is the negative input to the error amplifier.

The output switches to regulate this pin to 806mV with

respect to the exposed ground pad. Bias current flows

out of the FB pin.

ILIM1/2: The voltage present at the ILIM pin determines

the peak inductor current for the channel. The ILIM pin is

driven by an internal current source with a typical value

of 12µA. A resistor from the ILIM pin to ground sets the

ILIM voltage; the resistor value must be between 42.2k

and 120k. The maximum current limit range is 4.8A to

1.8A when the ILIM voltages are 1V and 0.5V respectively.

IND1/2: The IND pin is the input to the internal sense resistor

that measures current flowing in the inductor. When the

current in the resistor exceeds the current dictated by the

VC pin, the SW latch is held in reset, disabling the output

switch. Bias current flows out of the IND pin.

RT/SYNC: The voltage present at the RT/SYNC pin deter-

mines the constant switching frequency. The RT/SYNC

pin is driven by an internal current source with a typical

value of 12µA which allows a single resistor from the RT/

SYNC pin to ground to set the RT/SYNC voltage and result-

ing switching frequency. Minimum switching frequency

is typically 110kHz when VRT/SYNC is 0V and maximum

switching frequency is typically 2.5MHz when VRT/SYNC

is above 950mV.

Driving the RT/SYNC pin with an external clock signal will

synchronize the switch to the applied frequency. Synchro-

nization occurs on the rising edge of the clock signal after

the clock signal is detected. Each rising clock edge initiates

an oscillator ramp reset. A gain control loop servos the

oscillator charging current to maintain constant oscillator

amplitude. Hence, the slope compensation and channel

phase relationship remain unchanged. If the clock signal

is removed, the oscillator reverts to resistor mode after

the synchronization detection circuitry times out. The clock

source impedance should be set such that the current out

of the RT/SYNC pin in resistor mode generates a frequency

roughly equivalent to the synchronization frequency. See

the Applications Information section for more information.

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For more information www.linear.com/LT3992

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