LT1107_15 Datasheet, PDF (6/16 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1107
BLOCK DIAGRA S
LT1107
LT1107-5/ LT1107-12
SET
V_IN
1.25V
REFERENCE
GND
FB
A2
AO
GAIN BLOCK/
ERROR AMP
I_LIM
SW1
A1
OSCILLATOR
COMPARATOR
DRIVER
SW2
1107 BD01
SET
V_IN
1.25V
REFERENCE
R1
GND
A2
AO
GAIN BLOCK/
ERROR AMP
I_LIM
A1
OSCILLATOR
COMPARATOR
DRIVER
R2
220k
SENSE
LT1107-5: R1 = 73.5k
LT1107-12: R1 = 25.5k
SW1
SW2
1107 BD02
OPERATIO
The LT1107 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 LT1107 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 63kHz 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 11µs ON time and 5µs
OFF time in step-up mode, optimizing the device for
converters where V_OUT ≈ 3V_IN. The combination of high
duty cycle and the current limit feature enables continuous
mode operation in many applications, increasing available
output power.
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 LT1107-5 and LT1107-12 are functionally identical to
the LT1107. 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.
1107fa
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