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LTM4653

Rev 0

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PGOOD (D1): Power Good Indicator, Open-Drain Output

Pin. PGOOD is high impedance when PGDFB is within

approximately ±7.5% of 0.6V. PGOOD is pulled to GND

when PGDFB is outside this range.

PGDFB (D2): Power Good Feedback Programming Pin.

Connect PGDFB to VOSNS through a resistor, RPGDFB.

RPGDFB configures the voltage threshold of VOUT for which

PGOOD toggles its state. If the PGOOD feature is used,

set RPGDFB to:

RPGDFB =

VOUT

0.6V

– 1

⎛

⎝

⎜

⎞

⎠

⎟ • 4.99k

otherwise, leave PGDFB open circuit.

A small filter capacitor (220pF) internal to the LTM4653

on this pin provides high frequency noise immunity for

the PGOOD output indicator.

fSET (E3): Oscillator Frequency Programming Pin. The

default switching frequency of the LTM4653 is 400kHz.

Often, it is necessary to increase the programmed fre-

quency by connecting a resistor between fSET and SGND.

(See the Applications Information section.) Note that the

synchronization range of CLKIN is approximately ±40%

of the oscillator frequency programmed by the fSET pin.

CLKIN (B1): Mode Select and Oscillator Synchronization

Input. Leave CLKIN open circuit for forced continuous

mode operation. Alternatively, this pin can be driven to

synchronize the switching frequency of the LTM4653 to

a clock signal. In this condition, the LTM4653 operates

in forced continuous mode and the cycle-by-cycle turn-

on of the primary power MOSFET MT is coincident with

the rising edge of the clock applied to CLKIN. Note the

synchronization range of CLKIN is approximately ±40%

of the oscillator frequency programmed by the fSET pin.

(See the Applications Information section.)

COMPa (E2): Current Control Threshold and Error Ampli-

fier Compensation Node. The trip threshold of LTM4653’s

current comparator increases with a corresponding rise

in COMPa voltage. A small filter cap (10pF) internal to the

LTM4653 on this pin introduces a high-frequency roll-

off of the error-amplifier response, yielding good noise

rejection in the control-loop. COMPa is often electrically

connected to COMPb in one’s application, thus applying

default loop compensation. Loop compensation (a series

resistor-capacitor) can be applied externally to COMPa if

desired or needed, instead. (See COMPb.)

COMPb (E1): Internal Loop Compensation Network. For

mostapplications,theinternal,defaultloopcompensation

of the LTM4653 is suitable to apply “as is”, and yields very

satisfactoryresults:applythedefaultloopcompensationto

the control loop by simply connecting COMPa to COMPb.

When more specialized applications require a personal

touch to the optimization of control loop response, this

can be accomplished by connecting a series resistor-

capacitor network from COMPa to SGND—and leaving

COMPb open circuit.

VINREG (D3): Input Voltage Regulation Programming

Pin. Optionally connect this pin to the midpoint node

formed by a resistor-divider between VD and SGND. When

the voltage on VINREG falls below approximately 2V, a

VINREG control loop servos VOUT to decrease the power

inductor current and thus regulate VINREG at 2V. (See

the Applications Information section.)

If this input voltage regulation feature is not desired, con-

nect VINREG to INTVCC.

IMONa (C2): Power Inductor Current Analog Indicator Pin

and Current Limit Programming Pin. The current flowing

out of this pin is equal to 1/40,000 of the average power

inductor current. To construct a voltage (VIMONa) that is

proportional to the power inductor current, optionally

apply a parallel resistor-capacitor network to this pin and

terminate it to SGND.

IMONa can be connected to IMONb if the default resis-

tor-capacitor termination network provided by IMONb is

desired: 1V at full scale (4A) load current. (See IMONb.)

If this analog indicator feature is not desired, connect

IMONa to SGND.

If IMONa ever exceeds a trip threshold of approximately

2V, an IMON control loop servos VOUT to decrease power

inductor current and thus regulate IMONa at 2V. In this

manner, the average current limit inception threshold of

the LTM4653 can be configured. (See the Applications

Information section.)

PIN FUNCTIONS

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