LT1111 Datasheet, PDF (6/16 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1111
BLOCK DIAGRA S
LT1111
SET
V _IN
1.25V
REFERENCE
GND
FB
+
A2
A0
–
GAIN BLOCK/
ERROR AMP
I _LIM
SW1
+
A1
–
COMPARATOR
OSCILLATOR
DRIVER
SW2
LT1111 • BD01
LT1111-5/ LT1111-12
SET
V _IN
1.25V
REFERENCE
R1
+
A2
A0
–
GAIN BLOCK/
ERROR AMP
I _LIM
SW1
+
A1
OSCILLATOR
–
DRIVER
COMPARATOR
R2
SW2
220k
LT1111-5: R1 = 73.5k
SENSE
LT1111-12: R1 = 25.5k
GND
LT1111 • BD02
LT1111 OPERATI
The LT1111 is a gated oscillator switcher. This type
architecture has very low supply current because the
switch is cycled when the feedback pin voltage drops
below the reference voltage. Circuit operation can best be
understood by referring to the LT1111 block diagram.
Comparator A1 compares the feedback (FB) pin voltage
with the 1.25V reference signal. When FB drops below
1.25V, A1 switches on the 72kHz oscillator. The driver
amplifier boosts the signal level to drive the output NPN
power switch. The switch cycling action raises the output
voltage and FB pin voltage. When the FB voltage is suffi-
cient to trip A1, the oscillator is gated off. A small amount
of hysteresis built into A1 ensures loop stability without
external frequency compensation. When the comparator
output is low, the oscillator and all high current circuitry is
turned off, lowering device quiescent current to just 300µA.
The oscillator is set internally for 7µs ON time and 7µs OFF
time, optimizing the device for circuits where V_OUT and V_IN
differ by roughly a factor of 2. Examples include a 3V to 5V
step-up converter or a 9V to 5V step-down converter.
Gain block A2 can serve as a low-battery detector. The
negative input of A2 is the 1.25V reference. A resistor
divider from V_IN to GND, with the mid-point connected to
the SET pin provides the trip voltage in a low-battery
detector application. AO can sink 300µA (use a 22k
resistor pull-up to 5V).
A resistor connected between the I_LIM pin and V_IN sets
maximum switch current. When the switch current ex-
ceeds the set value, the switch cycle is prematurely
terminated. If current limit is not used, I_LIM should be tied
directly to V_IN. Propagation delay through the current limit
circuitry is approximately 1µs.
In step-up mode the switch emitter (SW2) is connected to
ground and the switch collector (SW1) drives the induc-
tor; in step-down mode the collector is connected to V_IN
and the emitter drives the inductor.
The LT1111-5 and LT1111-12 are functionally identical to
the LT1111. The -5 and -12 versions have on-chip voltage
setting resistors for fixed 5V or 12V outputs. Pin 8 on the
fixed versions should be connected to the output. No
external resistors are needed.
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