LTC4267CDHC datasheet(23/32 Pages) LINEAR

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LTC4267

4267fc

The shunt regulator can sink up to 5mA through the PVCC

pin to PGND. The values of RSTART and CPVCC must be

selected for the application to withstand the worst-case

load conditions and drop on PVCC, ensuring that the PVCC

turn-off threshold is not reached. CPVCC should be sized

sufﬁciently to handle the switching current needed to drive

NGATE while maintaining minimum switching voltage.

actual current needed to power the LTC4267 switching

regulator goes through Q1 and PVCC sources current on

an “as-needed” basis. The static current is then limited

only to the current through RB and D1.

APPLICATIO S I FOR ATIO

Figure 12. Powering the LTC4267 Switching

Regulator via the Shunt Regulator

Figure 13. Powering the LTC4267 Switching

Regulator with an External Preregulator

VPORTP

PVCC

PGND

POUT

VPORTN

LTC4267

–48

FROM

PSE

RSTART

CPVCC

–

PGND

4267 F14

External Preregulator

The circuit in Figure 13 shows a third way to power the

LTC4267 switching regulator circuit. An external series

preregulator consists of a series pass transistor Q1, zener

diode D1, and a bias resistor RB. The preregulator holds

PVCC at 7.6V nominal, well above the maximum rated PVCC

turn-off threshold of 6.8V. Resistor RSTART momentarily

charges the PVCC node up to the PVCC turn-on threshold,

enabling the switching regulator. The voltage on CPVCC

begins to decline as the switching regulator draws its

normal supply current, which exceeds the delivery of

RSTART. After some time, the output voltage approaches

the desired value. By this time, the pass transistor Q1

catches the declining voltage on the PVCC pin, and provides

virtually all the supply current required by the LTC4267

switching regulator. CPVCC should be sized sufﬁciently to

handle the switching current needed to drive NGATE while

maintaining minimum switching voltage.

The external preregulator has improved efﬁciency over

the simple resistor-shunt regulator method mentioned

previously. RB can be selected so that it provides a small

current necessary to maintain the zener diode voltage and

the maximum possible base current Q1 will encounter. The

VPORTP

PVCC

PGND

POUT

VPORTN

LTC4267

–48

FROM

PSE

RSTART

CPVCC

–

PGND

8.2V

4267 F15

Compensating the Main Loop

In an isolated topology, the compensation point is typically

chosen by the components conﬁgured around the external

error ampliﬁer. Shown in Figure 14, a series RC network

is connected from the compare voltage of the error am-

pliﬁer to the error ampliﬁer output. In PD designs where

transient load response is not critical, replace RZ with a

short. The product of R2 and CC should be sufﬁciently large

to ensure stability. When fast settling transient response

is critical, introduce a zero set by RZCC. The PD designer

must ensure that the faster settling response of the output

voltage does not compromise loop stability.

In a nonisolated design, the LTC4267 incorporates an

internal error ampliﬁer where the ITH/RUN pin serves as

a compensation point. In a similar manner, a series RC

network can be connected from ITH/RUN to PGND as

shown in Figure 15. CC and RZ are chosen for optimum

load and line transient response.

Figure 14. Main Loop Compensation for an Isolated Design

TO OPTO-

ISOLATOR

4267 F14

VOUT

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