LT1111CS8PBF LINEAR TECH, LT1111CS8PBF Datasheet - Page 6

• Manufacturer Part Number: LT1111CS8PBF
• Manufacturer: LINEAR TECH

LT1111
BLOCK
OPERATIO
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 300 A.
The oscillator is set internally for 7 s ON time and 7 s OFF
time, optimizing the device for circuits where V
differ by roughly a factor of 2. Examples include a 3V to 5V
step-up converter or a 9V to 5V step-down converter.
6
REFERENCE
1.25V
GND
V
IN
SET
I D AGRA
FB
U
+
+
–
–
GAIN BLOCK/
A2
ERROR AMP
COMPARATOR
A1
LT1111
W
S
OSCILLATOR
A0
DRIVER
I
OUT
LIM
LT1111 • BD01
and V
SW1
SW2
IN
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
the SET pin provides the trip voltage in a low-battery
detector application. AO can sink 300 A (use a 22k
resistor pull-up to 5V).
A resistor connected between the I
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
directly to V
circuitry is approximately 1 s.
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
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.
REFERENCE
1.25V
GND
V
IN
R1
SET
IN
. Propagation delay through the current limit
IN
to GND, with the mid-point connected to
220k
R2
+
+
–
–
LT1111-5/LT1111-12
GAIN BLOCK/
A2
ERROR AMP
COMPARATOR
A1
SENSE
LT1111-12:
OSCILLATOR
A0
LT1111-5:
LIM
LIM
pin and V
R1 = 73.5k
R1 = 25.5k
should be tied
DRIVER
I
LIM
LT1111 • BD02
IN
sets
1111fd
SW1
SW2
IN