LT1316 LINER [Linear Technology], LT1316 Datasheet - Page 6

• Manufacturer Part Number: LT1316
• Description: Micropower DC/DC Converter with Programmable Peak Current Limit
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
Table 1 simplifies component selection for commonly
used input and output voltages. The methods used in
determining these values are discussed in more detail later
in this data sheet.
V
Table 1. R
Operation
To understand operation of the LT1316, first examine
Figure 1. Comparator A1 monitors FB voltage which is
V
When voltage at the FB pin drops below the reference
voltage (1.23V), A1’s output goes high and the oscillator
is enabled. The oscillator has an off-time fixed at 2 s and
an on-time limited to 6.3 s. Power transistor Q1 is cycled
on and off by the oscillator forcing current through the
inductor to alternately ramp up and down (see Figure 2).
LT1316
6
OUT
V
OUT
AC COUPLED
2
2
2
2
5
5
5
5
IN
V
200mV/DIV
100mA/DIV
INDUCTOR
CURRENT
OUT
5V/DIV
can be set using the equation:
divided down by resistor divider network R1/R2.
V
V
12
28
28
28
V
OUT
5
5
5
5
OUT
SW
= 1.23
SET
Resistor and Inductor Values
CURRENT
100mA
10mA
25mA
50mA
75mA
10mA
LOAD
1mA
5mA
Figure 2. Switching Waveforms
R2 + R1
U
R2
RESISTOR
36.8k
18.2k
6.81k
6.81k
22.1k
INFORMATION
R
10k
75k
10k
10 s/DIV
U
SET
V
OUT
INDUCTOR
R1
R2
100 H
100 H
100 H
100 H
W
1316 EQF01
68 H
47 H
33 H
82 H
FB
1316 F02
PEAK SWITCH
CURRENT
U
165mA
320mA
500mA
490mA
140mA
270mA
80mA
56mA
During the portion of the switch cycle when Q1 is turned
off, current is forced through D1 to C1 causing output
voltage to rise. This switching action continues until
output voltage rises enough to overcome A1’s hysteresis.
Peak switch current is set by a resistor from the R
to ground. Voltage at the R
and is used to set up a constant current through R5. This
current also flows through R3 which sets the voltage at the
positive input of comparator A2. When Q1 turns on, the
SW pin goes low and current ramps up at the rate V
Current through Q2 is equal to Q1’s current divided by 200.
When current through Q2 causes the voltage drop across
R4 and R3 to be equal, A2 changes state and resets the
oscillator, causing Q1 to turn off. Shutdown is accom-
plished by grounding the SHDN pin.
The low-battery detector A3 has its own 1.17V reference
and is always on. The open collector output device can sink
up to 500 A. Approximately 35mV of hysteresis is built
into A3 to reduce “buzzing” as the battery voltage reaches
the trip level.
Current Limit
During active mode when the part is switching, current in
the inductor ramps up each switch cycle until reaching a
preprogrammed current limit. This current limit value
must be set by placing the appropriate resistor from the
R
using Figure 3 to locate the desired DC current limit and
SET
Figure 3. DC Current Limit vs R
Note: DC Current is the Peak Switch Current if the Power
Transistor had Zero Turn-Off Delay
pin to ground. This resistance value can be found by
1000
100
10
10
R
SET
SET
(k )
pin is forced to 0.5V by A4
SET
Resistor
100
1316 F03
SET
IN
pin
/L.