LT1573CS8 datasheet(12/16 Pages) LINER

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LT1573

needed during output transients, more capacitance can be

added to the regulator output. If more capacitance is

added to the output, the bandwidth of the regulator is

lowered. A large value compensation capacitor may be

needed to lower the frequency of the compensation zero to

avoid high frequency oscillation. Equal value output

capacitors with different ESR can have different output

transient response. High frequency performance will be

strongly affected by parasitics in the output capacitor and

board layout. Some experimentation with the external

compensation will be required for optimum results.

Shutdown Function

The regulator can be shut down by pulling the SHDN pin

voltage higher than the shutdown threshold (about 1.3V).

The regulator will restart itself if the SHDN is pulled below

the shutdown threshold.The SHDN pin should be tied to

ground if it is not used. The SHDN pin voltage can be

higher than the input voltage. When the SHDN pin voltage

is higher than 2V, the SHDN pin current increases and is

limited by a 20k resistor. Momentarily putting the device

into shutdown also resets the overcurrent latch.

Lower Dropout Voltage or Higher Output

Current Capability

Lower dropout voltage or higher output current capability

can be achieved by paralleling several output PNP transis-

tors as shown in Figure 3. By paralleling output PNP

transistors, the equivalent resistance between the emit-

ters (VIN) and collectors (VOUT) is lowered or each PNP

transistor sharing the output current now runs at a lower

collector current, which causes the dropout voltage to

decrease. Because the PNP transistors are running at a

lower collector current where the transistor beta is higher,

much more output current can be obtained at a given base

drive current. When paralleling two or more output tran-

sistors, a separate resistor is needed for RB and RD for

each output transistor. This allows the base drive current

to be split evenly between output transistors, which pro-

motes equal output current sharing. In the specific

example drawn in Figure 3 with two output transistors, the

resistance of RB1 and RB2 is now twice the value of the

resistance of RB in Figure 2, and the resistance of RD1 and

RD2 is twice the value of the resistance of RD in Figure 2.

In case of n PNP transistors in parallel, the resistance RB

COMP

VOUT

VIN

DRIVE

LATCH

SHDN

GND

LT1573

CTIME

COUT1

VIN

VOUT

GND

1573 F03

RB1

RB2

RD2

RD1

LOAD

QOUT2

QOUT1

CIN

Figure 3. Reduced Dropout Voltage or Increased Output Current

by Paralleling Output PNP Transistors

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