LT1107CN8-5 Linear Technology, LT1107CN8-5 Datasheet - Page 6

• Manufacturer Part Number: LT1107CN8-5
• Description: IC DC/DC CONV FIXED OUT 5V 8-DIP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Step-Up (Boost), Invertingr

Specifications of LT1107CN8-5

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 400mA
• Frequency - Switching: 63kHz
• Voltage - Input: 2 ~ 30 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Through Hole
• Package / Case: 8-DIP (0.300", 7.62mm)
• Power - Output: 500mW
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

OPERATIO
LT1107
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 300 A.
The oscillator is set internally for 11 s ON time and 5 s
OFF time in step-up mode, optimizing the device for
converters where V
duty cycle and the current limit feature enables continuous
mode operation in many applications, increasing available
output power.
BLOCK
6
REFERENCE
1.25V
GND
V
IN
SET
I D AGRA
FB
U
OUT
A2
GAIN BLOCK/
ERROR AMP
COMPARATOR
A1
LT1107
3V
W
IN
S
. The combination of high
OSCILLATOR
AO
DRIVER
I
LIM
1107 BD01
SW1
SW2
GND
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 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.
REFERENCE
1.25V
V
IN
R1
IN
SET
. Propagation delay through the current limit
IN
to GND, with the mid-point connected to
220k
R2
LT1107-5/LT1107-12
A2
GAIN BLOCK/
ERROR AMP
COMPARATOR
A1
SENSE
AO
OSCILLATOR
LT1107-5: R1 = 73.5k
LT1107-12: R1 = 25.5k
LIM
LIM
pin and V
should be tied
DRIVER
I
LIM
IN
sets
1107fa
1107 BD02
SW1
SW2
IN