Texas Instruments LM3404 Evaluation Boards LM3404FSTDIMEV/NOPB LM3404FSTDIMEV/NOPB 사용자 설명서
제품 코드
LM3404FSTDIMEV/NOPB
L
T
S
DT
S
t
I
LED(t)
I
F
V
OUT
L
V
IN
- V
OUT
'
i
L
D =
t
ON
t
ON
+ t
OFF
=
= t
ON
x f
SW
t
ON
T
S
Connecting to LED Array
4
Connecting to LED Array
The LM3402/04 evaluation board includes two standard 94 mil turret connectors for the cathode and
anode connections to a LED array.
anode connections to a LED array.
5
Low Power Shutdown
The LM3402/04 can be placed into a low power shutdown state (I
Q
typically 90 µA) by grounding the DIM
terminal. During normal operation this terminal should be left open-circuit.
6
Constant On Time Overview
The LM3402 and LM3404 are buck regulators with a wide input voltage range and a low voltage
reference. 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. With the addition of a PNP transistor, the on-
timer can be made to be inversely proportional to the input voltage minus the output voltage. This is one of
the application improvements made to this demonstration board that will be discussed later (improved
average LED current circuit).
reference. 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. With the addition of a PNP transistor, the on-
timer can be made to be inversely proportional to the input voltage minus the output voltage. This is one of
the application improvements made to this demonstration board that will be discussed later (improved
average LED current circuit).
The LM3402 / 04 were designed with a focus of controlling the current through the load, not the voltage
across it. A constant current regulator is free of load current transients, and has no need for output
capacitance to supply the load and maintain output voltage. Therefore, in this demonstration board in
order to demonstrate the fast transient capabilities, I have chosen to omit the output capacitor. With any
Buck regulator, duty cycle (D) can be calculated with the following equations.
across it. A constant current regulator is free of load current transients, and has no need for output
capacitance to supply the load and maintain output voltage. Therefore, in this demonstration board in
order to demonstrate the fast transient capabilities, I have chosen to omit the output capacitor. With any
Buck regulator, duty cycle (D) can be calculated with the following equations.
(1)
The average inductor current equals the average LED current whether an output capacitor is used or not.
Figure 3. Buck Converter Inductor Current Waveform
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
). A
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
.
3
SNVA342E – July 2008 – Revised April 2013
AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current
Evaluation Board
Copyright © 2008–2013, Texas Instruments Incorporated