LT1170/LT1171/LT1172
5
117012fi
For more information www.linear.com/LT1170
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
DC
MAX
Maximum Switch Duty Cycle
l
85 92 97 %
Shutdown Mode
Supply Current
3V ≤ V
IN
≤ V
MAX
V
C
= 0.05V
100 250 µA
Shutdown Mode
Threshold Voltage
3V ≤ V
IN
≤ V
MAX
l
100
50
150 250
300
mV
mV
Flyback Sense Delay Time (Note 5) 1.5 µs
elecTrical characTerisTics
The l denotes the specifications which apply over the full operating
temperature 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.
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: Minimum effective switch “on” time for the LT1170/LT1171/
LT1172 (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 +
R
(
)
I
L
+ Vf
t
(
f
(
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.
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.
