LT1573C View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1573C' PDF : 16 Pages View PDF
LT1573
APPLICATIO S I FOR ATIO
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
+
CTIME
VIN
FB
COMP
LATCH
VOUT
LT1573
SHDN
VIN
GND
DRIVE
CC
RC
RB1
RB2
QOUT1
QOUT2
RD1
RD2
+
R1
CIN
COUT1
R2
VOUT
LOAD
GND
1573 F03
Figure 3. Reduced Dropout Voltage or Increased Output Current
by Paralleling Output PNP Transistors
12
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