LTC1704EGN#TRPBF【PDF 24/28 页】IC REG DL BUCK/LINEAR 16SSOP

LTC1704/LTC1704B

24

1704bfa

APPLICATIO S I FOR ATIO

U

doesnt cause a bigger spike than the transient signal

being measured. Conveniently, the typical probe tip ground

clip is spaced just right to span the leads of a typical output

capacitor. Make sure the bandwidth limit on the scope is

turned off, since a significant portion of the transient

energy occurs above the 20MHz cutoff.

Now that we know how to measure the signal, we need to

have something to measure. The ideal situation is to use

the actual load for the test, and switch it on and off while

watching the output. If this isnt convenient, a current step

generator is needed. This generator needs to be able to

turn on and off in nanoseconds to simulate a typical

switching logic load, so stray inductance and long clip

leads between the LTC1704 and the transient generator

must be minimized.

Figure 12 shows an example of a simple transient genera-

tor. Be sure to use a noninductive resistor as the load

elementmany power resistors use an inductive spiral

pattern and are not suitable for use here. A simple solution

is to take ten 1/4W film resistors and wire them in parallel

to get the desired value. This gives a noninductive resistive

load which can dissipate 2.5W continuously or 50W if

pulsed with a 5% duty cycle, enough for most LTC1704

circuits. Solder the MOSFET and the resistor(s) as close to

the output of the LTC1704 circuit as possible and set up

the signal generator to pulse at a 100Hz rate with a 5% duty

cycle. This pulses the LTC1704 with 500祍 transients

10ms apart, adequate for viewing the entire transient

recovery time for both positive and negative transitions

while keeping the load resistor cool.

LINEAR REGULATOR SUPPLY

Linear Regulator Output Voltage

The linear regulator senses the output voltage at V

OUTREG

with an internal amplifier (see Figure 13). The amplifier

negative input is connected internally to an 800mV refer-

ence, while the positive input is connected to the REGFB

pin. The amplifier output drives a P-channel transistor

MREG, which is in turn connected to the external NPN pass

transistor. The linear regulator output voltage can be

obtained using the following equation:

V

R

OUTREG

= +

?/DIV>

08 1

5

6

.

Figure 12. Transient Load Generator

LTC1704

V

OUTSW

IRFZ44 OR

EQUIVALENT

R

LOAD

50&

0V TO 10V

100Hz, 5%

DUTY CYCLE

LOCATE CLOSE TO THE OUTPUT

1704 F12

PULSE

GENERATOR

Figure 13. Linear Regulator

QEXT

MOFF

R5

R6

REGDR

REGILM

MREG

V

CC

V

REF

2mA

REGFB

1704 F13

+

C

OUTREG

V

OUTREG

V

REGON

R

REGILM

C

DELAY

I

P

V

INREG

REGOFF

V

CC

1.9礎

LTC1704

REG

ILM

REGFB

+

AMP

Linear Regulator Supplies Requirement

The linear regulator operates with two supplies: V

CC

for the

LTC1704 and V

INREG

for the external NPN transistor QEXT.

Both supplies must be higher than the minimum value

determined by the linear regulator output voltage, V

OUTREG

.

For a desired V

OUTREG

, use the following formula to

calculate the minimum required V

CC

:

Minimum V

CC

= V

OUTREG

+ V

BE(QEXT)

+ V

DROPOUT

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