Datasheet
f
SW
=
V
O
1.34 x 10
-10
x R
ON
V
O
= n x V
F
+ 200 mV
LM3402/02HV
CS
V
SNS
I
F
R
SNS
I
F
+
-
VREF
+
-
One-shot
CS
Comparator
V
O
V
F
V
F
LED 1
LED n
t
ON
= 1.34 x 10
-10
x
R
ON
V
IN
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:
(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.
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:
(2)
V
F
= forward voltage of each LED, n = number of LEDs in series (3)
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