LT1956IGN datasheet(22/28 Pages) LINER

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LT1956/LT1956-5

1956f

APPLICATIO S I FOR ATIO

oscillations. If needed, an additional capacitor (CF) can be

added across the RC/CC network from the VC pin to ground

to further suppress VC ripple voltage.

With a tantalum output capacitor, the LT1956 already

includes a resistor (RC) and filter capacitor (CF) at the VC

pin (see Figures 10 and 11) to compensate the loop over

the entire VIN range (to allow for stable pulse skipping for

high VIN-to-VOUT ratios ≥ 4). A ceramic output capacitor

can still be used with a simple adjustment to the resistor

RC for stable operation (see Ceramic Capacitors section

for stabilizing LT1956). If additional phase margin is

required, a capacitor (CFB) can be inserted between the

output and FB pin but care must be taken for high output

voltage applications. Sudden shorts to the output can

create unacceptably large negative transients on the FB

pin.

For VIN-to-VOUT ratios < 4, higher loop bandwidths are

possible by readjusting the frequency compensation com-

ponents at the VC pin.

When checking loop stability, the circuit should be oper-

ated over the application’s full voltage, current and tem-

perature range. Proper loop compensation may be obtained

by empirical methods as described in Application Notes 19

and 76.

CONVERTER WITH BACKUP OUTPUT REGULATOR

In systems with a primary and backup supply, for ex-

ample, a battery powered device with a wall adapter input,

the output of the LT1956 can be held up by the backup

supply with the LT1956 input disconnected. In this condi-

tion, the SW pin will source current into the VIN pin. If the

SHDN pin is held at ground, only the shut down current of

µA will be pulled via the SW pin from the second supply.

With the SHDN pin floating, the LT1956 will consume its

quiescent operating current of 1.5mA. The VIN pin will also

source current to any other components connected to the

input line. If this load is greater than 10mA or the input

could be shorted to ground, a series Schottky diode must

be added, as shown in Figure 12. With these safeguards,

the output can be held at voltages up to the VIN absolute

maximum rating.

BUCK CONVERTER WITH ADJUSTABLE SOFT-START

Large capacitive loads or high input voltages can cause

high input currents at start-up. Figure 13 shows a circuit

that limits the dv/dt of the output at start-up, controlling

the capacitor charge rate. The buck converter is a typical

configuration with the addition of R3, R4, CSS and Q1.

As the output starts to rise, Q1 turns on, regulating switch

FREQUENCY (Hz)

–20

–40

180

150

120

1956 F11

GAIN

PHASE

VIN = 12V

VOUT = 5V

ILOAD = 500mA

COUT = 100µF, 10V, 0.1Ω

10k

100

100k

RC = 2.2k

CC = 22nF

CF = 220pF

1.22V

LT1956

GND

1956 F10

OUTPUT

ESR

200k

ERROR

AMPLIFIER

RLOAD

CURRENT MODE

POWER STAGE

gm = 2mho

gm =

2000

µmho

TANTALUM

ESL

CERAMIC

CFB

Figure 10. Model for Loop Response

Figure 11. Overall Loop Response

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