Datasheet

4
LT1170/LT1171/LT1172
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
∆I
IN
Supply Current Increase 25 35 mA/A
∆I
SW
During Switch On-Time
f Switching Frequency 88 100 112 kHz
● 85 115 kHz
DC
MAX
Maximum Switch Duty Cycle ● 85 92 97 %
Shutdown Mode 3V ≤ V
IN
≤ V
MAX
100 250 µA
Supply Current V
C
= 0.05V
Shutdown Mode 3V ≤ V
IN
≤ V
MAX
100 150 250 mV
Threshold Voltage
● 50 300 mV
Flyback Sense Delay Time (Note 5) 1.5 µs
Transformer designs will tolerate much higher input voltages because
leakage inductance limits rate of rise of current in the switch. These
designs must be evaluated individually to assure that current limit is well
controlled up to maximum input voltage.
Boost mode designs are never protected against output shorts because
the external catch diode and inductor connect input to output.
Note 3: Measured with V
C
in hi clamp, V
FB
= 0.8V. I
SW
= 4A for LT1170,
2A for LT1171, and 1A for LT1172.
Note 4: For duty cycles (DC) between 50% and 80%, minimum
guaranteed switch current is given by I
LIM
= 3.33 (2 – DC) for the LT1170,
I
LIM
= 1.67 (2 – DC) for the LT1171, and I
LIM
= 0.833 (2 – DC) for the
LT1172.
Note 5: Minimum input voltage for isolated flyback mode is 7V. V
MAX
=
55V for HV grade in fully isolated mode to avoid switch breakdown.
Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
Note 2: Minimum effective switch “on” time for the LT1170/71/72 (in current
limit only) is ≈ 0.6µs. This limits the maximum safe input voltage during an
output shorted condition. Buck mode and inverting mode input voltage during
an output shorted condition is limited to:
V
IN
(max, output shorted) = 15V +
buck and inverting mode
R = Inductor DC resistance
I
L
= 10A for LT1170, 5A for LT1171, and 2.5A for LT1172
Vf = Output catch diode forward voltage at I
L
t = 0.6µs, f = 100kHz switching frequency
Maximum input voltage can be increased by increasing R or Vf.
External current limiting such as that shown in AN19, Figure 39, will
provide protection up to the full supply voltage rating. C1 in Figure 39
should be reduced to 200pF.
Switch Current Limit vs Duty Cycle*
TEMPERATURE (°C)
–75
MINIMUM INPUT VOLTAGE (V)
–25
25
50 150
1170/1/2 G02
–50 0
75
100
125
2.9
2.8
2.7
2.6
2.5
2.4
2.3
SWITCH CURRENT = 0A
SWITCH CURRENT = I
MAX
Minimum Input Voltage Switch Saturation Voltage
DUTY CYCLE (%)
0
SWITCH CURRENT (A)
16
12
8
4
0
40
1170/1/2 G01
20
30
50
60
70
80 90 100
25°C
125°C
–55°C
10
* DIVIDE VERTICAL SCALE BY TWO FOR
LT1171, BY FOUR FOR LT1172.
SWITCH CURRENT (A)*
0
SWITCH SATURATION VOLTAGE (V)
8
1170/1/2 G03
1
2
3
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
4567
25°C
150°C
–55°C
100°C
* DIVIDE CURRENT BY TWO FOR
LT1171, BY FOUR FOR LT1172.
(R)(I
L
) + Vf
(t)(f)
The ● denotes the specifications which apply over the full operating tem-
perature range, otherwise specifications are at T
A
= 25°C. V
IN
= 15V, V
C
= 0.5V, V
FB
= V
REF
, output pin open, unless otherwise noted.
TYPICAL PERFOR A CE CHARACTERISTICS
UW
ELECTRICAL CHARACTERISTICS