LTC1149 查看數據表（PDF） - Linear Technology

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LTC1149

High Efficiency Synchronous Step-Down Switching Regulators

Linear Technology 

TEST CIRCUIT

LTC1149

LTC1149-3.3/LTC1149-5

+

0.1µF

VIN

0.068µF

IRF9Z34

MBR380

+

220µF

100V

390pF

0.047µF

+

1µF

3300pF

+

1k

V8

1

PGATE

16

CAP

2

VIN

SHDN2 15

3

VCC

RGND 14

4

PDRIVE

NGATE 13

5

LTC1149

12

VCC

PGND

6

CT

SGND 11

7

ITH

8 SENSE–

VFB/ 10

SHDN1

SENSE+ 9

1000pF

+

V15

IRFZ34

50µH

25k

+

V9 – V8

75k +

220µF

+

V10

0.05Ω

VOUT

1149 TC

UU W U

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

7

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