Datasheet

3
LT1937
1937f
TYPICAL PERFOR A CE CHARACTERISTICS
UW
Current Limit vs Duty Cycle
Quiescent Current SHDN Pin Bias Current
Switching Frequency
Feedback Bias Current
UU
U
PI FU CTIO S
SW (Pin 1): Switch Pin. Connect inductor/diode here.
Minimize trace area at this pin to reduce EMI.
GND (Pin 2): Ground Pin. Connect directly to local ground
plane.
FB (Pin 3): Feedback Pin. Reference voltage is 95mV.
Connect cathode of lowest LED and resistor here. Calcu-
late resistor value according to the formula:
R
FB
= 95mV/I
LED
SHDN (Pin 4): Shutdown Pin. Connect to 1.5V or higher to
enable device; 0.4V or less to disable device.
GND (Pin 5, SC70 Package): Ground Pin. Connect to Pin
2 and to local ground plane
V
IN
(Pin 5/Pin 6 SC70 Package): Input Supply Pin. Must
be locally bypassed.
V
IN
(V)
0
0
I
Q
(mA)
0.4
0.8
1.2
1.6
2468
1937 G01
10
2.0
0.2
0.6
1.0
1.4
1.8
2.2
–50°C
100°C
25°C
TEMPERATURE (°C)
–50
SHDN PIN BIAS CURRENT (µA)
350
25
1937 G02
200
100
–25 0 50
50
0
400
300
250
150
75 100
SHDN = 10V
SHDN = 3.6V SHDN = 3V
SHDN = 2.7V
TEMPERATURE (°C)
–50
0.8
1.0
1.4
25 75
1937 G03
0.6
0.4
–25 0
50 100
0.2
0
1.2
SWITCHING FREQUENCY (MHz)
TEMPERATURE (°C)
–50
FEEDBACK BIAS CURRENT (nA)
40
50
60
25 75
1937 G04
30
20
–25 0
50 100
10
0
Efficiency vs Temperature
TEMPERATURE (°C)
–50
EFFICIENCY (%)
82
83
1937 G05
81
80
0
50
100
85
84
I
LED
= 20mA
I
LED
= 15mA
I
LED
= 10mA
V
IN
= 3.6V
3 LEDs
DUTY CYCLE (%)
0
350
300
250
200
150
100
50
0
60
1937 G06
20 40 80 100
CURRENT LIMIT (mA)