Texas Instruments LM3404 Evaluation Boards LM3404HVEVAL/NOPB LM3404HVEVAL/NOPB Benutzerhandbuch
Produktcode
LM3404HVEVAL/NOPB
f
SW
=
V
O
1.34 x 10
-10
x R
ON
V
O
= n x V
F
+ 200 mV
LM3404/04HV
CS
R
SNS
One-shot
CS
Comparator
V
O
V
F
I
F
LED 1
LED n
+
-
-
+
I
F
V
SNS
V
REF
t
ON
= 1.34 x 10
-10
x
R
ON
V
IN
SNVS465F – OCTOBER 2006 – REVISED MAY 2013
APPLICATION INFORMATION
THEORY OF OPERATION
The LM3404 and LM3404HV 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 1.2A. 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 LM3404/04HV.
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 1.2A. 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 LM3404/04HV.
CONTROLLED ON-TIME OVERVIEW
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 LM3404/04HV regulators should be operated in continuous conduction mode (CCM), where inductor current
stays positive throughout the switching cycle. During steady-state CCM operation, the converter maintains a
constant switching frequency that can be selected using the following equation:
stays positive throughout the switching cycle. During steady-state CCM operation, the converter maintains a
constant switching frequency that can be selected using the following equation:
•
V
F
= forward voltage of each LED
•
n = number of LEDs in series
(2)
Copyright © 2006–2013, Texas Instruments Incorporated
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