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LTC1149

LTC1149-3.3/LTC1149-5

TEST CIRCUIT

APPLICATIO S I FOR ATIO

Typical Application Circuit

The basic LTC1149 series application circuit is shown in

Figure 1. External component selection is driven by the

input voltage and output load requirement, and begins

with the selection of RSENSE. Once RSENSE is known, CT

and L can be chosen. Next, the power MOSFETs and D1

are selected. Finally, CIN and COUT are selected and the

loop is compensated. The circuit shown in Figure 1 can be

configured for operation up to an input voltage of 48V. If

the application does not require greater than 15V opera-

tion, then the LTC1148 should be used.

RSENSE Selection for Output Current

RSENSE is chosen based on the required output current.

The LTC1149 series current comparator has a threshold

range which extends from a minimum of 25mV/RSENSE to

a maximum of 150mV/RSENSE. The current comparator

threshold sets the peak of the inductor ripple current,

yielding a maximum output current IMAX equal to the peak

value less half the peak-to-peak ripple current.

For proper

Burst Mode operation, IRIPPLE(P-P) must be less than or

equal to the minimum current comparator threshold.

Since efficiency generally increases with ripple current,

the maximum allowable ripple current is assumed, i.e.,

IRIPPLE(P-P) = 25mV/RSENSE (see CT and L Selection for

Operating Frequency). Solving for RSENSE and allowing a

margin for variations in the LTC1149 series and external

component values yields:

RSENSE =

100mV

IMAX

A graph for selecting RSENSE versus maximum output

current is given in Figure 2. The LTC1149 series works well

with values of RSENSE from 0.02Ω to 0.2Ω.

The load current below which Burst Mode operation

commences, IBURST, and the peak short-circuit current,

ISC(PK), both track IMAX. Once RSENSE has been chosen,

IBURST and ISC(PK) can be predicted from the following

equations:

IBURST ≈

15mV

RSENSE

ISC(PK) =

150mV

RSENSE

PGATE

VIN

VCC

PDRIVE

VCC

ITH

SENSE–

CAP

SHDN2

RGND

NGATE

PGND

SGND

SENSE+

LTC1149

0.047

µF

0.068

µF

VIN

0.1

µF

3300pF

390pF

1000pF

V9 – V8

25k

75k

220

µF

0.05

Ω

V10

VOUT

V15

µH

IRF9Z34

220

µF

100V

IRFZ34

MBR380

1149 TC

VFB/

SHDN1

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