LM3404EVAL National Semiconductor, LM3404EVAL Datasheet - Page 10

• Manufacturer Part Number: LM3404EVAL
• Description: BOARD, EVALUATION, FOR LM3404
• Manufacturer: National Semiconductor

Specifications of LM3404EVAL

• Kit Contents: Board, Datasheet
• Svhc: No SVHC (15-Dec-2010)
• Features: Thermal Shutdown Protection, Separate PWM Dimming And Low Power Shutdown
• Kit Features: Cycle-by-Cycle Current Limit, Separate PWM

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Block Diagram
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 con-
trol 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 pro-
tection round out the functions of the LM3404/04HV.
CONTROLLED ON-TIME OVERVIEW
Figure 1
rent through an array of LEDs. A voltage signal, V
created as the LED current flows through the current setting
resistor, R
where it is compared against a 200 mV reference, V
on-comparator turns on the power MOSFET when V
below V
on-time, t
voltage, V
REF
shows the feedback system used to control the cur-
ON
IN
SNS
. The power MOSFET conducts for a controlled
, set by an external resistor, R
. On-time is governed by the following equation:
, to ground. V
SNS
is fed back to the CS pin,
ON
, and by the input
REF
SNS
SNS
. The
falls
, is
10
At the conclusion of t
minimum off-time, t
plete the CS comparator compares V
waiting to begin the next cycle.
The LM3404/04HV regulators should be operated in contin-
uous conduction mode (CCM), where inductor current stays
positive throughout the switching cycle. During steady-state
FIGURE 1. Comparator and One-Shot
OFF-MIN
ON
the power MOSFET turns off for a
, of 300 ns. Once t
SNS
and V
20205405
OFF-MIN
REF
20205403
is com-
again,