LT1110
6
LT
1110
U
OPER
O
AT
I
LT
1110
The LT1110 is a gated oscillator switcher. This type
architecture has very low supply current because the
switch is cycled only when the feedback pin voltage drops
below the reference voltage. Circuit operation can best be
understood by referring to the LT1110 block diagram
above. Comparator A1 compares the FB pin voltage with
the 220mV reference signal. When FB drops below
220mV, A1 switches on the 70kHz oscillator. The driver
amplifier boosts the signal level to drive the output NPN
power switch Q1. An adaptive base drive circuit senses
switch current and provides just enough base drive to
ensure switch saturation without overdriving the switch,
resulting in higher efficiency. The switch cycling action
raises the output voltage and FB pin voltage. When the FB
voltage is sufficient 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 is low the oscillator and all high current
circuitry is turned off, lowering device quiescent current to
just 300µA for the reference, A1 and A2.
The oscillator is set internally for 10µs ON time and 5µs
OFF time, optimizing the device for step-up circuits where
V
OUT
≈ 3V
IN
, e.g., 1.5V to 5V. Other step-up ratios as well
as step-down (buck) converters are possible at slight
losses in maximum achievable power output.
A2 is a versatile gain block that can serve as a low battery
detector, a linear post regulator, or drive an under voltage
lockout circuit. The negative input of A2 is internally
connected to the 220mV reference. An external resistor
divider from V
IN
to GND provides the trip point for A2. The
AO output can sink 300µA (use a 47k resistor pull up to
+5V). This line can signal a microcontroller that the battery
voltage has dropped below the preset level. To prevent the
gain block from operating in its linear region, a 2MΩ
resistor can be connected from AO to SET. This provides
positive feedback.
A resistor connected between the I
LIM
pin and V
IN
adjusts
maximum switch current. When the switch current ex-
ceeds the set value, the switch is turned off. This feature
is especially useful when small inductance values are used
with high input voltages. If the internal current limit of 1.5A
is desired, I
LIM
should be tied directly to V
IN
. Propagation
delay through the current limit circuitry is about 700ns.
In step-up mode, SW2 is connected to ground and SW1
drives the inductor. In step-down mode, SW1 is con-
nected to V
IN
and SW2 drives the inductor. Output voltage
is set by the following equation in either step-up or step-
down modes where R1 is connected from FB to GND and
R2 is connected from V
OUT
to FB.
The LT1110-5 and LT1110-12 fixed output voltage ver-
sions have the gain setting resistors on-chip. Only three
external components are required to construct a 5V or 12V
output converter. 16µA flows through R1 and R2 in the
LT1110-5, and 39µA flows in the LT1110-12. This current
represents a load and the converter must cycle from time
to time to maintain the proper output voltage. Output
ripple, inherently present in gated oscillator designs, will
typically run around 90mV for the LT1110-5 and 200mV
for the LT1110-12 with the proper inductor/capacitor
selection. This output ripple can be reduced considerably
by using the gain block amp as a pre-amplifier in front of
the FB pin. See the Applications section for details.
W
IDAGRA
B
L
O
C
K
-5, -12
LT
1110
A1
LT1110 • BD02
IN
V
GND
SET
AO
A2
220mV
REF
OSCILLATOR
DRIVER
+
–
R1
SW1
SW2
LIM
I
R2
300kΩ
SENSE
LT1110-5:
LT1110-12:
R1 = 13.8kΩ
R2 = 5.6kΩ
GAIN BLOCK/ERROR AMP
COMPARATOR
Q1
U
OPER
O
AT
I
-5, -12
VmV
R
R
OUT
=
()
+
220
2
1
101.()
