LTC1627 查看數據表（PDF） - Linear Technology

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LTC1627

Monolithic Synchronous Step-Down Switching Regulator

Linear Technology 

LTC1627

APPLICATIONS INFORMATION

Checking Transient Response

The regulator loop response can be checked by looking at

the load transient response. Switching regulators take

several cycles to respond to a step in load current. When

a load step occurs, VOUT immediately shifts by an amount

equal to (∆ILOAD • ESR), where ESR is the effective series

resistance of COUT. ∆ILOAD also begins to charge or dis-

charge COUT, which generates a feedback error signal. The

regulator loop then acts to return VOUT to its steady-state

value. During this recovery time VOUT can be monitored for

overshoot or ringing that would indicate a stability prob-

lem. The internal compensation provides adequate com-

pensation for most applications. But if additional compen-

sation is required, the ITH pin can be used for external

compensation as shown in Figure 7.

A second, more severe transient is caused by switching in

loads with large (>1µF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in parallel

with COUT, causing a rapid drop in VOUT. No regulator can

deliver enough current to prevent this problem if the load

switch resistance is low and it is driven quickly. The only

solution is to limit the rise time of the switch drive so that

the load rise time is limited to approximately (25 • CLOAD).

Thus, a 10µF capacitor would require a 250µs rise time,

limiting the charging current to about 130mA.

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC1627. These items are also illustrated graphically in

the layout diagram of Figure 7. Check the following in your

layout:

1. Are the signal and power grounds segregated? The

LTC1627 signal ground consists of the resistive

divider, the optional compensation network (RC and

CC1), CSS and CC2. The power ground consists of the

(–) plate of CIN, the (–) plate of COUT and Pin 4 of the

LTC1627. The power ground traces should be kept

short, direct and wide. The signal ground and power

ground should converge to a common node in a star-

ground configuration.

2. Does the VFB pin connect directly to the feedback

resistors? The resistive divider R1/R2 must be con-

nected between the (+) plate of COUT and signal ground.

CC2

OPTIONAL

RC

CC1

CSS

1

ITH

SYNC/FCB 8

2

RUN/SS

VDR 7

LTC1627

3

VFB

VIN 6

4

GND

SW 5

BOLD LINES INDICATE

HIGH CURRENT PATHS

L1

+

R2

+

VOUT

COUT

R1

–

OPTIONAL

CV

+

+

D1

CIN

VIN

D2

CB

–

1627 F07

Figure 7. LTC1627 Layout Diagram

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