LM3402HVMR Datasheet, PDF (11/38 Pages) Texas Instruments – 0.5A Constant Current Buck Regulator for Driving High Power LEDs

LM3402, LM3402HV
www.ti.com
SNVS450E – SEPTEMBER 2006 – REVISED MAY 2013
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 control 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 protection round out the functions of the LM3402/02HV.
CONTROLLED ON-TIME OVERVIEW
Figure 19 shows the feedback system used to control the current 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:
t_ON
=
1.34
x
10^-10
x
R_ON
V_IN
(1)
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
complete the CS comparator compares V_SNS and V_REF again, waiting to begin the next cycle.
V_O
LED 1
V_F
LM3402/02HV ^IF
CS
Comparator
One-shot
-
+
VREF ₊
-
LED n
V_F
CS
V_SNS
I_F
R_SNS
Figure 19. Comparator and One-Shot
The LM3402/02HV regulators should be operated in continuous conduction mode (CCM), where inductor current
stays positive throughout the switching cycle. During steady-state operationin the CCM, the converter maintains
a constant switching frequency, which can be selected using the following equation:
V_O
f_SW = _{1.34 x 10-10 x RON}
V_O = n x V_F + 200 mV
(2)
V_F = forward voltage of each LED, n = number of LEDs in series
(3)
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