LT1316 View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LT1316 Datasheet PDF : 16 Pages
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LT1316
APPLICATIONS INFORMATION
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.
VOUT can be set using the equation:
VOUT
) VOUT = 1.23
R2 + R1
R2
R1
FB
R2
1316 EQF01
Table 1. RSET Resistor and Inductor Values
LOAD
RSET
VIN VOUT CURRENT RESISTOR INDUCTOR
25
10mA
36.8k
100µH
25
25mA
18.2k
68µH
25
50mA
10k
47µH
25
75mA
6.81k
33µH
5 12 100mA
6.81k
82µH
5 28
1mA
75k
100µH
5 28
5mA
22.1k
100µH
5 28 10mA
10k
100µH
PEAK SWITCH
CURRENT
80mA
165mA
320mA
500mA
490mA
56mA
140mA
270mA
Operation
To understand operation of the LT1316, first examine
Figure 1. Comparator A1 monitors FB voltage which is
VOUT 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).
VOUT
AC COUPLED
200mV/DIV
VSW
5V/DIV
INDUCTOR
CURRENT
100mA/DIV
10µs/DIV
1316 F02
Figure 2. Switching Waveforms
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 RSET pin
to ground. Voltage at the RSET 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 VIN/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
RSET pin to ground. This resistance value can be found by
using Figure 3 to locate the desired DC current limit and
1000
100
10
10
100
RSET (kΩ)
1316 F03
Figure 3. DC Current Limit vs RSET Resistor
Note: DC Current is the Peak Switch Current if the Power
Transistor had Zero Turn-Off Delay
6
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