LTM8045 datasheet(27/54 Pages) AD

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LTM4655

Rev. 0

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

connecting ISET

na to ISETnb. For other ramp-up rates,

connect an external CSS capacitor parallel to RISET.

When starting up into a pre-biased VOUT, the LTM4655

stays in a sleep mode, keeping MTn and MBn off until

VISETna equals VOSNSn+—after which, the DC/DC con-

verter commences switching action and VOUT is ramped

according to the voltage commanded by ISET

na.

Since the LTM4655 control loop servos its VOSNSn+ volt-

age to match that of ISET

na’s, the LTM4655’s channel

n output can be configured to track any voltage applied

to ISET

na, referenced to SVOUTn–. See Figure 52 for an

example of the LTM4655 configured as a DAC-controlled

bipolar-output programmable power supply.

The LTM4655 can track the mirror-image of a posi-

tive rail to generate the negative half of a split-sup-

ply, as seen in Figure 50 (note the use of RTRACK and

RISET2 = RISET1 || RTRACK).

Frequency Adjustment

The default switching frequency (fSWn) of channel n of the

LTM4655 is 400kHz. This is suitable for low-VIN (VINn ≤

5V) applications and low-VOUT (VOUTn+ – VOUTn– ≤ 3.3V)

applications. For a practical design, the LTM4655’s induc-

tor ripple current (∆InPK-PK) is suggested to be less than

~2APK-PK. Choose fSWn according to Equation 11.

fSWn =

VOUTn+ − VOUTn− • 1−Dn

(

)

Ln • ∆InPK-PK

(11)

where the value of LTM4655’s power inductor, Ln, is 4μH.

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

ensure minimum on-time criteria is met (Equation 12).

fSWn <

TONn(MIN)

(12)

RfSETn NOT USED

RfSETn (kΩ)

100

10k

0.1

4655 F01

Figure 1. Relationship Between RfSETn and Target fSWn

The LTM4655’s minimum on-time, tONn(MIN), is specified

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

guardband to 90ns.

To configure channel

n of the LTM4655 for a higher

switching frequency than its default of 400kHz, apply

a resistor, RfSETn, between the fSETn pin and SVOUTn+.

RfSETn is given (in MΩ) by Equation 13.

RfSETn(MΩ)=

10pF •[fSWn(MHz)–0.4(MHz)]

(13)

The relationship of RfSETn to programmed fSWn is shown

in Figure 1. See Table 11 and Table 12 for recommended

fSWn and corresponding RfSETn values for various com-

binations of VINn, VOUTn+ and VOUTn–.

OPERATION

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