4

LT1937

1937f

–

+

RQ

S

0.2

Ω

SW

DRIVER

COMPARATOR

2

SHDN

4

1

3

FB

95mV

5

–

+

Σ

RAMP

GENERATOR

1.2MHz

OSCILLATOR

GND

1937 BD1

Q1

A2

A1

SHUTDOWN

1.25V

(PIN 6 FOR

SC70 PACKAGE)

(PINS 2 AND 5 FOR

SC70 PACKAGE)

BLOCK DIAGRA

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

Figure 2. LT1937 Block Diagram

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 300

Ω resistor across

the inductor, although this will degrade efficiency.

5V/DIV

50mA/DIV

100mV/DIV

0.2

µs/DIV

1937 F03