LT1076_15 Datasheet, PDF (9/16 Pages) Linear Technology – Step-Down Switching Regulator

LT1074/LT1076
PI DESCRIPTIO S
current is well controlled even when switch duty cycle
must be extremely low. Theoretical switch “on” time for a
buck converter in continuous mode is:
t_ON
=
V_OUT
V_IN
+ V_D
•f
V_D = Catch diode forward voltage ( ≈ 0.5V)
f = Switching frequency
At f = 100kHz, t_ON must drop to 0.2µs when V_IN = 25V
and the output is shorted (V_OUT = 0V). In current limit,
the LT1074 can reduce t_ON to a minimum value of
≈0.6µs, much too long to control current correctly for
V_OUT = 0. To correct this problem, switching frequency
is lowered from 100kHz to 20kHz as the FB pin drops
from 1.3V to 0.5V. This is accomplished by the circuitry
TO
OSCILLATOR
+2V
Q1
V_OUT
R1
+ 2.21V R3
ERROR
3k
AMPLIFIER
V_C
–
EXTERNAL
DIVIDER
FB
R2
2.21k
Figure 3. Frequency Shifting
LT1074•PD03
shown in Figure 3.
Q1 is off when the output is regulating (V_FB = 2.21V). As
the output is pulled down by an overload, V_FB will eventu-
ally reach 1.3V, turning on Q1. As the output continues to
drop, Q1 current increases proportionately and lowers the
frequency of the oscillator. Frequency shifting starts when
the output is ≈ 60% of normal value, and is down to its
minimum value of ≅ 20kHz when the output is ≅ 20% of
normal value. The rate at which frequency is shifted is
determined by both the internal 3k resistor R3 and the
external divider resistors. For this reason, R2 should not
be increased to more than 4kΩ, if the LT1074 will be
subjected to the simultaneous conditions of high input
voltage and output short-circuit.
SHUTDOWN PIN
The shutdown pin is used for undervoltage lockout, micro-
power shutdown, soft-start, delayed start, or as a general
purpose on/off control of the regulator output. It controls
switching action by pulling the I_LIM pin low, which forces
the switch to a continuous “off” state. Full micropower
shutdown is initiated when the shutdown pin drops below
0.3V.
The V/I characteristics of the shutdown pin are shown in
Figure 4. For voltages between 2.5V and ≈V_IN, a current of
10µA flows out of the shutdown pin. This current in-
creases to ≈25µA as the shutdown pin moves through the
2.35V threshold. The current increases further to ≈30µA at
the 0.3V threshold, then drops to ≈15µA as the shutdown
voltage fall below 0.3V. The 10µA current source is in-
cluded to pull the shutdown pin to its high or default state
when left open. It also provides a convenient pull-up for
delayed start applications with a capacitor on the shut-
down pin.
When activated, the typical collector current of Q1 in
Figure 5, is ≈2mA. A soft-start capacitor on the I_LIM pin will
delay regulator shutdown in response to C1, by
≈(5V)(C_LIM)/2mA. Soft-start after full micropower shut-
down is ensured by coupling C2 to Q1.
0
–5
T_j = 25°C
CURRENT FLOWS OUT
OF SHUTDOWN PIN
–10
–15
SHUTDOWN
–20
THRESHOLD
–25
–30
–35
–40
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
VOLTAGE (V)
LT1074•PC05
Figure 4. Shutdown Pin Characteristics
sn1074 1074fds
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