LT1108CS8-5 Linear Technology, LT1108CS8-5 Datasheet - Page 5

• Manufacturer Part Number: LT1108CS8-5
• Description: IC DC/DC CONV FIXED OUT 5V 8SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Step-Up (Boost), Invertingr

Specifications of LT1108CS8-5

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 400mA
• Frequency - Switching: 19kHz
• Voltage - Input: 2 ~ 30 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Power - Output: 500mW
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

1 OPER
The LT1108 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 LT1108 block diagram.
Comparator A1 compares the feedback (FB) pin voltage
with the 1.245V reference signal. When FB drops below
1.245V, A1 switches on the 19kHz 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
110 A.
The oscillator is set internally for 36 s ON-time and 17 s
OFF-time, allowing continuous mode operation in many
cases such as 2V to 5V converters. Continuous mode
greatly increases available output power.
Gain block A2 can serve as a low battery detector. The
BLOCK
REFERENCE
1.245V
GND
V
IN
ATI
SET
I D AGRA
FB
UO
A2
GAIN BLOCK/
ERROR AMP
COMPARATOR
A1
LT1108
W
S
OSCILLATOR
A0
DRIVER
LT1108 • BD
I
LIM
SW1
SW2
negative input of A2 is the 1.245V reference. A resistor
divider from V
the SET pin provides the trip voltage in a low battery
detector application. A0 can sink 100 A (use a 47k resis-
tor 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
limit circuitry is approximately 2 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 LT1108-5 and LT1108-12 are functionally identical to
the LT1108. 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.
GND
REFERENCE
1.245V
V
IN
R1
IN
SET
IN
. Propagation delay through the current-
to GND, with the mid-point connected to
753k
R2
LT1108-5/LT1108-12
SENSE
A2
GAIN BLOCK/
ERROR AMP
COMPARATOR
A1
LT1108-5: R1 = 250k
LT1108-12: R1 = 87.4k
OSCILLATOR
LIM
A0
LIM
pin and V
should be tied
LT1108
LT1108-5 • BD
DRIVER
I
LIM
IN
sets
5
SW1
SW2
IN