LT1939
6
1939f
TYPICAL PERFORMANCE CHARACTERISTICS
Boost Current vs Switch Current
Minimum Boost Voltages vs
Temperature
Minimum Input Voltage
LDRV Dropout Voltage vs
Temperature
Inductor Value for 2A Maximum
Load Current (V
OUT
= 3.3V,
I
RIPPLE
= 250mA)
V
IN
(Pin 1): The V
IN
pin powers the internal control circuitry
and is monitored by an undervoltage comparator. The V
IN
pin is also connected to the collectors of the internal power
NPN switch and linear output NPN. The V
IN
pin has high
dI/dt edges and must be decoupled to ground close to
the pin of the device.
SHDN (Pin 2): The SHDN pin is used to shut down the
LT1939 and reduce quiescent current to a typical value
of 12糀. The accurate 0.76V threshold and input current
hysteresis can be used as an undervoltage lockout, prevent-
ing the regulator from operating until the input voltage has
reached a predetermined level. Force the SHDN pin above
its threshold or let it oat for normal operation.
SS (Pin 3): The SS pin is used to control the slew rate
of the output of both the switching and linear regulators.
A single capacitor from the SS pin to ground determines
the regulators ramp rate. For soft-start details see the
Applications Information section.
Switcher Dropout Operation
SWITCH CURRENT (A)
0
30
40
50
1.6
1939 G19
20
10
25
35
45
15
5
0
0.4
0.2
0.8
0.6
1.2 1.4 1.8
1.0
2.0
150癈
25癈
50癈
TEMPERATURE (癈)
50
1.5
1.7
1.9
2.1
2.3
0
50
100
150
1939 G20
2.5
2.7
25
25
75
125
MINIMUM BOOST FOR
SWITCH SATURATION
LOAD CURRENT (A)
0
8
7
6
5
4
3
2
1
0
1.6
1939 G21
0.4 0.8 1.2
2.0
1.4
0.2 0.6 1.0
1.8
V
OUT1
= 5V
V
OUT1
= 3.3V
F
SW
= 1MHz
L = 3.3糎
TEMPERATURE (癈)
50
1.00
1.05
1.15
1.20
1.25
1.50
1.35
0
50 75
1939 G22
1.10
1.40
1.45
1.30
25
25
100
125
150
I
LDRV
= 5mA
INPUT VOLTAGE (V)
2.5
0
1
2
3
4
6
3.0 3.5 4.0 4.5
1939 G23
5.0 5.5
5
I
OUT1
= 1A
V
OUT1
= 5V
V
OUT1
= 3.3V
INPUT VOLTAGE (V)
5
250
500
1000
1250
1500
2500
1939 G24
750
15
10
20
25
1750
2000
2250
L = 3.3糎
L = 4.7糎
L = 6.8糎
L = 10糎
L = 1.5糎
L = 2.2糎
L = 1糎
PIN FUNCTIONS
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