LT1937
W
BLOCK DIAGRA
V
IN
FB
3
(PIN 6 FOR
SC70 PACKAGE)
5
1
SW
–
+
COMPARATOR
A2
–
+
A1
DRIVER
95mV
V
REF
Q1
R
Q
R
C
1.25V
S
C
C
+
–
0.2Ω
Σ
RAMP
GENERATOR
SHDN
4
(PINS 2 AND 5 FOR
SC70 PACKAGE)
SHUTDOWN
2
GND
1937 BD1
1.2MHz
OSCILLATOR
Figure 2. LT1937 Block Diagram
U
OPERATIO
pulses. This will result in some low frequency ripple,
althoughtheLED currentremainsregulatedonanaverage
basis down to zero. The photo in Figure 3 details circuit
operation driving three white LEDs at a 4mA load. Peak
inductor current is less than 50mA and the regulator
operates in discontinuous mode, meaning the inductor
current reaches zero during the discharge phase. After the
inductor current reaches zero, the switch pin exhibits
ringing due to the LC tank circuit formed by the inductor
in combination with switch and diode capacitance. This
ringingisnotharmful;farlessspectralenergyiscontained
in the ringing than in the switch transitions. The ringing
can be damped by application of a 300Ω resistor across
the inductor, although this will degrade efficiency.
The LT1937 uses a constant frequency, current mode
control scheme to provide excellent line and load regula-
tion. Operation can be best understood by referring to the
block diagram in Figure 2. At the start of each oscillator
cycle, the SR latch is set, which turns on the power switch
Q1. A voltage proportional to the switch current is added
to a stabilizing ramp and the resulting sum is fed into the
positive terminal of the PWM comparator A2. When this
voltageexceedsthelevelatthenegativeinputofA2,theSR
latch is reset turning off the power switch. The level at the
negative input of A2 is set by the error amplifier A1, and is
simply an amplified version of the difference between the
feedback voltage and the reference voltage of 95mV. In
this manner, the error amplifier sets the correct peak
current level to keep the output in regulation. If the error
amplifier’s output increases, more current is delivered to
the output; if it decreases, less current is delivered.
VSW
5V/DIV
Minimum Output Current
IL2
The LT1937 can regulate three series LEDs connected at
low output currents, down to approximately 4mA from a
4.2V supply, without pulse skipping, using the same
external components as specified for 15mA operation. As
current is further reduced, the device will begin skipping
50mA/DIV
VOUT
100mV/DIV
0.2µs/DIV
1937 F03
Figure 3. Switching Waveforms at ILED = 4mA, VIN = 3.6V
1937f
4
Linear [ Linear ]
