Datasheet

6
LT1316
APPLICATIONS INFORMATION
WUU
U
10µs/DIV
1316 F02
Figure 2. Switching Waveforms
R
SET
(kΩ)
10
DC CURRENT LIMIT (mA)
100
1000
10 100
1316 F03
Figure 3. DC Current Limit vs R
SET
Resistor
Note: DC Current is the Peak Switch Current if the Power
Transistor had Zero Turn-Off Delay
V
OUT
AC COUPLED
200mV/DIV
V
SW
5V/DIV
INDUCTOR
CURRENT
100mA/DIV
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
SET
pin
to ground. Voltage at the R
SET
pin is forced to 0.5V by A4
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
IN
/L.
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 500µA. 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
SET
pin to ground. This resistance value can be found by
using Figure 3 to locate the desired DC current limit and
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
OUT
can be set using the equation:
V
OUT
= 1.23
R2 + R1
R2
) )
R1
V
OUT
R2
1316 EQF01
FB
Table 1. R
SET
Resistor and Inductor Values
LOAD R
SET
PEAK SWITCH
V
IN
V
OUT
CURRENT RESISTOR INDUCTOR CURRENT
2 5 10mA 36.8k 100µH 80mA
2 5 25mA 18.2k 68µH 165mA
2 5 50mA 10k 47µH 320mA
2 5 75mA 6.81k 33µH 500mA
5 12 100mA 6.81k 82µH 490mA
5 28 1mA 75k 100µH 56mA
5 28 5mA 22.1k 100µH 140mA
5 28 10mA 10k 100µH 270mA
Operation
To understand operation of the LT1316, first examine
Figure 1. Comparator A1 monitors FB voltage which is
V
OUT
divided down by resistor divider network R1/R2.
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 2µs and
an on-time limited to 6.3µs. 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).