LT1573 Datasheet, PDF (9/16 Pages) Linear Technology – Low Dropout PNP Regulator Driver

LT1573
APPLICATIO S I FOR ATIO
Resistor R_B helps to turn off the PNP (Q_OUT in Figure 2).
Smaller values for R_B turn off the PNP faster but will
increase input current. The recommended value for R_B is
50Ω. For circuits that do not require high output current or
fast transient response, the value of R_B can be increased
up to 200Ω. For the D45H11, the emitter-base voltage is
a function of base and collector current. Table 2 lists some
useful operating points for the D45H11. These points were
empirically determined using a sampling of devices.
Table 2. D45H11 V_BE
I_B
I_C
(mA)
(A)
1
0.2
7
1
23
2
45
3
66
4
100
5
V_BE AT 25°C
(V)
0.65
0.75
0.80
0.85
0.90
0.95
Design Example
Given the following operating requirements:
4.5V < V_IN < 5.5V
I_OUT(MAX) = 5A
V_OUT = 3.3V
1. The first step is to determine the required drive current
for the D45H11. Dropout voltage must be less than 1.2V
at 5A output current. From Table 1, a drive current of
100mA will give 0.7V dropout voltage at an output
current of 5A. This satisfies the operating require-
ments.
2. The next step is to determine the value of R_D. Assume
R_B is 50Ω. From Table 2, the maximum emitter-base
voltage for this design is 0.95V. The current through
R_B is:
I_RB = V_BE/R_B = 0.95/50 = 19mA
V_DRIVE is the DRIVE pin saturation voltage when the
DRIVE pin current equals 119mA, which can be read
from the typical performance characteristics curve to
be 0.39V. Resistor R_D now can be calculated from
Eq (1):
R_D = (4.5 – 0.95 – 0.39)V/(100 + 19)mA = 26.6Ω
The next lowest 5% value is 24Ω.
Overcurrent Latch-Off
In addition to limiting the base drive current, the resistor
R_D is included in the circuit for the overcurrent protection
latch-off function. There is a minimum value for this
resistance. It is calculated by Equation 1 with the drive
current I_DRIVE set to the minimum available drive current
(= 250mA) from the LT1573. At high currents, R_D also
limits the power dissipation in the LT1573. In some
conditions, resistor R_D can be replaced with a short. This
is possible in circuits where an overload is unlikely and the
input voltage and drive requirements are low. If resistor R_D
is not included in the circuit, the regulator is protected
against the overcurrent condition only by the thermal
shutdown function. After the resistor R_D is determined, a
certain amount of base drive current is available to the
external PNP transistor. An overcurrent or output short
condition will demand a base drive current greater than the
LT1573 can supply. The internal drive transistor will
saturate. A time-out latch will be triggered by this
overcurrent condition to turn off the regulator system. The
time-out period is determined by an external capacitor
connected between the LATCH and GND pins. The time-
out period is equal to the time it takes for the capacitor to
charge from 0V to the latch threshold which is equal to
2V_BE. The latch charging current is set by an internal
current source and is a function of input voltage and
temperature as shown in the typical performance charac-
teristics curve. At 25°C, the typical latch charging current
ranges from 7.2µA with 3V input to 8µA with 7V input. If
the overcurrent or output short condition persists longer
than the time-out period, the regulator will be shut down.
Otherwise, the regulator will function normally. In the
latch-off mode, some extra current is drawn from the input
to maintain the latch. The latching current is a function of
input voltage and temperature as shown in the typical
performance characteristic curve. At 25°C, the typical
latching current ranges from 0.3mA with 3V input to
9.5mA with 7V input. The latch can be reset by recycling
input power, by grounding the LATCH pin or by putting the
device into shutdown.
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