LTM4651 datasheet(9/34 Pages) LINER

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LTM4651

4651f

For more information www.linear.com/LTM4651

In addition, the output of the LTM4651 can track a voltage

applied between the ISETa pin and the SVOUT– pins. See

the Applications Information section.

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

Pin. PGOOD is high impedance when PGDFB – SVOUT– is

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

GND when PGDFB – SVOUT– is outside this range.

PGDFB (D2): Power Good Feedback Programming Pin.

Connect PGDFB to GNDSNS 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 LTM4651

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 LTM4651 is 400kHz.

Often, it is necessary to increase the programmed fre-

quency by connecting a resistor between fSETandSVOUT–.

(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):OscillatorSynchronizationInput.LeaveCLKIN

open circuit for forced continuous mode operation.

Alternatively,thispincanbedrivensoastosynchronizethe

switching frequency of the LTM4651 to a clock signal. In

thiscondition,theLTM4651operatesinforced-continuous

mode and the cycle-by-cycle turn-on of the Primary MOS-

FET is coincident with the rising edge of the clock applied

to CLKIN. Note the synchronization range of CLKIN is ap-

proximately±40%oftheoscillatorfrequencyprogrammed

by the fSET pin. See the Applications Information section.

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

fier Compensation Node. The trip threshold of LTM4651’s

current comparator increases with a respective rise in

COMPa voltage. A small filter capacitor (10pF) internal

to the LTM4651 on this pin introduces a high-frequency

roll-offoftheerror-amplifierresponse,yieldinggoodnoise

rejection in the control-loop. COMPa is usually 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 a majority of applications, the internal, default loop

compensation of the LTM4651 is suitable to apply “as is”

and yields very satisfactory results: apply the default loop

compensation to the control loop by simply connecting

COMPa to COMPb. When more specialized applications

requireapersonaltouchtotheoptimizationofcontrolloop

response,thiscanbeaccomplishedbyconnectingaseries

resistor-capacitor network from COMPa to SVOUT–—and

leaving COMPb open circuit.

VINREG(D3):InputVoltageRegulationProgrammingPin.

Optionally connect this pin to the midpoint node formed

by a resistor-divider between VD and SVOUT–. When the

voltage on VINREG falls below approximately 2V with

respect to SVOUT–, a VINREG control loop servos COMPa

so as to decrease the power inductor current and thus

regulate VINREG at 2V with respect to SVOUT–. See the

Applications Information section.

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

nect VINREG to INTVCC.

TEMP+ (J1, J6): Temperature Sensor, Positive Input.

Emitter of a 2N3906-genre PNP bipolar junction transistor

(BJT). Optionally interface to temperature monitoring cir-

cuitry such as LTC®2997, LTC2990, LTC2974 or LTC2975.

Otherwise leave electrically open. Pins J1 and J6 are

electrically connected together internal to the LTM4651,

and thus it is only necessary to connect one TEMP+ pin

to monitoring circuitry. The remaining TEMP+ pin can be

used for redundant connectivity or routed to an ICT test

point for design-for-test considerations, as desired.

PIN FUNCTIONS

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