LM3402/LM3402HV
Block Diagram
20192103
Application Information
THEORY OF OPERATION
The LM3402 and LM3402HV are buck regulators with a wide
input voltage range, low voltage reference, and a fast output
enable/disable function. These features combine to make
them ideal for use as a constant current source for LEDs with
forward currents as high as 500 mA. The controlled on-time
(COT) architecture is a combination of hysteretic mode con-
trol and a one-shot on-timer that varies inversely with input
voltage. Hysteretic operation eliminates the need for small-
signal control loop compensation. When the converter runs in
continuous conduction mode (CCM) the controlled on-time
maintains a constant switching frequency over the range of
input voltage. Fast transient response, PWM dimming, a low
power shutdown mode, and simple output overvoltage pro-
tection round out the functions of the LM3402/02HV.
CONTROLLED ON-TIME OVERVIEW
Figure 1 shows the feedback system used to control the cur-
rent through an array of LEDs. A voltage signal, V
SNS
, is
created as the LED current flows through the current setting
resistor, R
SNS
, to ground. V
SNS
is fed back to the CS pin,
where it is compared against a 200 mV reference, V
REF
. The
on-comparator turns on the power MOSFET when V
SNS
falls
below V
REF
. The power MOSFET conducts for a controlled
on-time, t
ON
, set by an external resistor, R
ON
, and by the input
voltage, V
IN
. On-time is governed by the following equation:
At the conclusion of t
ON
the power MOSFET turns off for a
minimum off-time, t
OFF-MIN
, of 300 ns. Once t
OFF-MIN
is com-
plete the CS comparator compares V
SNS
and V
REF
again,
waiting to begin the next cycle.
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