LM3402MRX National Semiconductor, LM3402MRX Datasheet - Page 10

• Manufacturer Part Number: LM3402MRX
• Description: LM3402/LM3402HV 0.5A Constant Current Buck Regulator for Driving High Power LEDs; Package: PSOP; No of Pins: 8; Qty per Container: 2500; Container: Reel
• Manufacturer: National Semiconductor

www.national.com
Block Diagram
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
10
created as the LED current flows through the current setting
resistor, R
where it is compared against a 200 mV reference, V
on-comparator turns on the power MOSFET when V
below V
on-time, t
voltage, V
At the conclusion of t
minimum off-time, t
plete the CS comparator compares V
waiting to begin the next cycle.
REF
ON
IN
SNS
. The power MOSFET conducts for a controlled
, set by an external resistor, R
. On-time is governed by the following equation:
, to ground. V
OFF-MIN
ON
the power MOSFET turns off for a
, of 300 ns. Once t
SNS
is fed back to the CS pin,
SNS
ON
, and by the input
and V
OFF-MIN
REF
20192103
REF
SNS
is com-
again,
. The
falls