LTM4613 datasheet(15/32 Pages) LINER

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LTM4653

Rev 0

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OPERATION

approximately –1A, allowing for significant negative aver-

age current. Clocking the CLKIN pin at a frequency within

±40% of the target switching frequency commanded by

the fSET pin synchronizes MT’s turn-on to the rising edge

of the CLKIN pin.

Output Voltage Programming, Tracking and Soft-Start

The LTM4653 regulates its output voltage, VOUT, accord-

ing to the differential voltage present across ISETa and

SGND. In most applications, the output voltage is set by

simply connecting a resistor, RISET, from ISETa to SGND,

according to:

RISET =

VOUT

50µA

Since the LTM4653 control loop servos its output volt-

age according to the voltage between ISETa and SGND:

placing a capacitor, CSS, parallel to RSET configures the

ramp-up rate of ISETa and thus VOUT. In the time domain,

the output voltage ramp-up after the RUN pin is toggled

from low to high (t = 0s) is given by:

VOUT(t)=IISETa •RISET • 1– e

–

RISET • CSS

⎛

⎝

⎜

⎜⎜

⎞

⎠

⎟

⎟⎟

The soft-start time, tSS, is defined as the time it takes for

VOUT to ramp from 0V to 90% of its final value:

tSS = –RISET •CSS •In (1– 0.9)

tSS = 2.3 • RISET •CSS

A default value of CSS = 1.5nF can be implemented by

connecting ISETa to ISETb. For other ramp-up rates,

connect an external CSS capacitor parallel to RISET. When

starting up into a pre-biased VOUT, the LTM4653 stays in

a sleep mode, keeping MT and MB off until VISETa equals

VOSNS—after which, the DC/DC converter commences

switching action and VOUT is ramped according to the

voltage commanded by ISETa.

Since the LTM4653 control loop servos its VOSNS voltage

to match that of ISETa’s, the LTM4653’s output can be

configured to track any voltage applied to ISETa, refer-

enced to SGND.

Frequency Adjustment

The default switching frequency (fSW) of the LTM4653

is 400kHz. This is suitable for low-VIN (VIN ≤ 5V) ap-

plications and low-VOUT (VOUT≤3.3V) applications. For a

practical design, the LTM4653’s inductor ripple current

(∆IPK-PK) is suggested to be less than ~2APK-PK. Choose

fSW according to:

fSW =

VOUT • 1− D

(

)

L • ∆IPK-PK

where the value of LTM4653’s power inductor, L, is 4μH.

To avoid cycle-skipping, impose restrictions on fSW, to

ensure minimum on time criteria is met:

fSW <

TON(MIN)

The LTM4653’s minimum on-time, tON(MIN), is specified

as 60ns. For a practical design, it is recommended to

guardband to 90ns.

ToconfiguretheLTM4653forahigherswitchingfrequency

than its default of 400kHz, apply a resistor, RfSET, between

the fSET pin and SGND. RfSET is given (in MΩ) by:

RfSET(MΩ)=

10pF •[fSW(MHz)– 0.4(MHz)]

The relationship of RfSET to programmed fSW is shown

in Figure 1. See Table 7 for recommended fSW and cor-

responding RfSET values for various combinations of VIN

and VOUT.

RfSET NOT USED

RfSET (kΩ)

100

10k

0.1

4653 F01

Figure 1. Relationship Between RfSET and Target fSW

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