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LM3404 数据表(PDF) 11 Page - Texas Instruments

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部件名 LM3404
功能描述  1.0A Constant Current Buck Regulator for Driving High Power LEDs
Download  39 Pages
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制造商  TI1 [Texas Instruments]
网页  http://www.ti.com
标志 TI1 - Texas Instruments

LM3404 数据表(HTML) 11 Page - Texas Instruments

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IL-MIN =
0.2
RSNS
L
-
VO x tSNS
fSW =
VO
1.34 x 10
-10 x RON
VO = n x VF + 200 mV
LM3404/04HV
CS
RSNS
One-shot
CS
Comparator
VO
VF
IF
LED 1
LED n
+
-
-
+
IF
VSNS
VREF
tON = 1.34 x 10
-10 x
RON
VIN
LM3404, LM3404HV
www.ti.com
SNVS465E – OCTOBER 2006 – REVISED FEBRUARY 2010
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.
CONTROLLED ON-TIME OVERVIEW
Figure 19 shows the feedback system used to control the current through an array of LEDs. A voltage signal,
VSNS, is created as the LED current flows through the current setting resistor, RSNS, to ground. VSNS is fed back
to the CS pin, where it is compared against a 200 mV reference, VREF. The on-comparator turns on the power
MOSFET when VSNS falls below VREF. The power MOSFET conducts for a controlled on-time, tON, set by an
external resistor, RON, and by the input voltage, VIN. On-time is governed by the following equation:
(1)
At the conclusion of tON the power MOSFET turns off for a minimum off-time, tOFF-MIN, of 300 ns. Once tOFF-MIN is
complete the CS comparator compares VSNS and VREF 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:
(2)
VF = forward voltage of each LED, n = number of LEDs in series
AVERAGE LED CURRENT ACCURACY
The COT architecture regulates the valley of
ΔVSNS, the AC portion of VSNS. To determine the average LED
current (which is also the average inductor current) the valley inductor current is calculated using the following
expression:
(3)
Copyright © 2006–2010, Texas Instruments Incorporated
Submit Documentation Feedback
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Product Folder Links: LM3404 LM3404HV


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