LTC1147I 查看數據表（PDF） - Linear Technology

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LTC1147I

High Efficiency Step-Down Switching Regulator Controllers

Linear Technology 

LTC1147-3.3

LTC1147-5/LTC1147L

APPLICATIO S I FOR ATIO

The basic LTC1147 application circuit is shown in Figure

1. External component selection is driven by the load

requirement and begins with the selection of RSENSE. Once

RSENSE is known, CT and L can be chosen. Next, the power

MOSFET 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 16V. If the application requires higher input

voltage, then the synchronous switched LTC1149 should

be used. Consult factory for lower minimum input voltage

version.

RSENSE Selection for Output Current

RSENSE is chosen based on the required output current.

The LTC1147 series current comparator has a thresh-

old 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 LTC1147 series and

external component values yields:

RSENSE

=

100mV

IMAX

A graph for selecting RSENSE versus maximum output

current is given in Figure 2.

The load current below in 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:

IBURST

≈

15mV

RSENSE

ISC(PK) =

150mV

RSENSE

0.20

0.15

0.10

0.05

0

0

1

2

3

4

5

MAXIMUM OUTPUT CURRENT (A)

LTC1147 • F02

Figure 2. Selecting RSENSE

The LTC1147 series automatically extend tOFF during a

short circuit to allow sufficient time for the inductor

current to decay between switch cycles. The resulting

ripple current causes the average short-circuit current

ISC(AVG) to be reduced to approximately IMAX.

L and CT Selection for Operating Frequency

The LTC1147 series use a constant off-time architecture

with tOFF determined by an external timing capacitor CT.

Each time the P-channel MOSFET switch turns on, the

voltage on CT is reset to approximately 3.3V. During the

off-time, CT is discharged by a current which is propor-

tional to VOUT. The voltage on CT is analogous to the

current in inductor L, which likewise decays at a rate

proportional to VOUT. Thus the inductor value must track

the timing capacitor value.

The value of CT is calculated from the desired continuous

mode operating frequency:

) CT

=

1

(1.3)(104)(f)

VIN – VOUT

VIN + VD

Where VD is the drop across the Schottky diode.

A graph for selecting CT versus frequency including the

effects of input voltage is given in Figure 3.

As the operating frequency is increased the gate charge

losses will reduce efficiency (see Efficiency Consider-

ations). The complete expression for operating frequency

sn1147 1147fds

7

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