LT1170/LT1171/LT1172
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating tem-
perature range, otherwise specifications are at TA = 25°C. VIN = 15V, VC = 0.5V, VFB = VREF, output pin open, unless otherwise noted.
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
∆I
∆I
Supply Current Increase
During Switch On-Time
25
35
mA/A
IN
SW
f
Switching Frequency
88
85
100
112
115
kHz
kHz
●
●
DC
Maximum Switch Duty Cycle
85
92
97
%
MAX
Shutdown Mode
Supply Current
3V ≤ V ≤ V
V = 0.05V
C
100
250
µA
IN
MAX
Shutdown Mode
Threshold Voltage
3V ≤ V ≤ V
100
50
150
250
300
mV
mV
IN
MAX
●
Flyback Sense Delay Time (Note 5)
1.5
µs
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
outputshortedcondition.Buckmodeandinvertingmodeinputvoltageduring
an output shorted condition is limited to:
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.
(R)(I ) + Vf
Note 3: Measured with V in hi clamp, V = 0.8V. I = 4A for LT1170,
2A for LT1171, and 1A for LT1172.
L
C
FB
SW
V
(max, output shorted) = 15V +
IN
(t)(f)
buck and inverting mode
R = Inductor DC resistance
Note 4: For duty cycles (DC) between 50% and 80%, minimum
guaranteed switch current is given by I = 3.33 (2 – DC) for the LT1170,
LIM
I = 10A for LT1170, 5A for LT1171, and 2.5A for LT1172
L
I
= 1.67 (2 – DC) for the LT1171, and I = 0.833 (2 – DC) for the
LIM
LIM
Vf = Output catch diode forward voltage at I
LT1172.
L
t = 0.6µs, f = 100kHz switching frequency
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.
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.
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Current Limit vs Duty Cycle*
Minimum Input Voltage
Switch Saturation Voltage
16
12
8
2.9
2.8
2.7
2.6
2.5
2.4
2.3
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
SWITCH CURRENT = I
MAX
150°C
100°C
25°C
–55°C
25°C
–55°C
125°C
SWITCH CURRENT = 0A
4
* DIVIDE VERTICAL SCALE BY TWO FOR
LT1171, BY FOUR FOR LT1172.
* DIVIDE CURRENT BY TWO FOR
LT1171, BY FOUR FOR LT1172.
0
70
0
10 20 30 40 50 60
80 90 100
2
4
5
6
7
0
1
3
8
–75 –50 –25
0
25 50 75 100 125 150
DUTY CYCLE (%)
SWITCH CURRENT (A)*
TEMPERATURE (°C)
1170/1/2 G01
1170/1/2 G02
1170/1/2 G03
4
Linear [ Linear ]
