LT1937
BLOCK DIAGRA
V
IN
(PIN 6 FOR
SC70 PACKAGE) 5
V
REF
1.25V
95mV
A1
+
R
C
C
C
Σ
–
SHDN
4
SHUTDOWN
1.2MHz
OSCILLATOR
RAMP
GENERATOR
(PINS 2 AND 5 FOR
SC70 PACKAGE)
Figure 2. LT1937 Block Diagram
OPERATIO
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
voltage exceeds the level at the negative input of A2, the SR
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.
Minimum Output Current
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
pulses. This will result in some low frequency ripple,
although the LED current remains regulated on an average
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
ringing is not harmful; far less spectral energy is contained
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.
V
SW
5V/DIV
I
L2
50mA/DIV
V
OUT
100mV/DIV
0.2µs/DIV
1937 F03
Figure 3. Switching Waveforms at I
LED
= 4mA, V
IN
= 3.6V
1937f
4
+
–
W
FB
3
1 SW
COMPARATOR
DRIVER
A2
R
S
Q
Q1
–
+
0.2Ω
2 GND
1937 BD1
U
LINER [ LINEAR TECHNOLOGY ]
