LM3405AXMYEVAL National Semiconductor, LM3405AXMYEVAL Datasheet - Page 6

• Manufacturer Part Number: LM3405AXMYEVAL
• Description: BOARD EVAL FOR LM3405 8MSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3405AXMYEVAL

• Current - Output / Channel: 750mA
• Outputs And Type: 1, Non-Isolated
• Features: Dimmable
• Voltage - Input: 5 ~ 18V
• Utilized Ic / Part: LM3405
• Kit Contents: Board, Datasheet
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: Cycle-by-Cycle Current Limit,
• Rohs Compliant: No
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

www.national.com
Block Diagram
Application Information
THEORY OF OPERATION
The LM3405A is a PWM, current-mode controled buck
switching regulator designed to provide a simple, high effi-
ciency solution for driving LEDs with a preset switching fre-
quency of 1.6MHz. This high frequency allows the LM3405A
to operate with small surface mount capacitors and inductors,
resulting in LED drivers that need only a minimum amount of
board space. The LM3405A is internally compensated, sim-
ple to use, and requires few external components.
The following description of operation of the LM3405A will re-
fer to the Simplified Block Diagram (Figure 1) and to the
waveforms in Figure 2. The LM3405A supplies a regulated
output current by switching the internal NMOS power switch
at constant frequency and variable duty cycle. A switching
cycle begins at the falling edge of the reset pulse generated
by the internal oscillator. When this pulse goes low, the output
control logic turns on the internal NMOS power switch. During
this on-time, the SW pin voltage (V
mately V
slope. I
generates an output proportional to the switch current. The
sense signal is summed with the regulator’s corrective ramp
and compared to the error amplifier’s output, which is propor-
tional to the difference between the feedback voltage and
V
ternal power switch turns off until the next switching cycle
begins. During the switch off-time, inductor current dis-
charges through the catch diode D1, which forces the SW pin
to swing below ground by the forward voltage (V
catch diode. The regulator loop adjusts the duty cycle (D) to
maintain a constant output current (I
forcing FB pin voltage to be equal to V
REF
. When the PWM comparator output goes high, the in-
L
IN
is measured by the current sense amplifier, which
, and the inductor current (I
SW
L
F
) increases with a linear
) swings up to approxi-
) through the LED, by
REF
(0.205V).
FIGURE 1. Simplified Block Diagram
D1
) of the
6
BOOST FUNCTION
Capacitor C3 and diode D2 in Figure 1 are used to generate
a voltage V
the gate drive voltage to the internal NMOS power switch. To
properly drive the internal NMOS switch during its on-time,
V
value of V
ficiency by minimizing both the internal switch ON resistance
(R
V
of 5.5V.
BOOST
BOOST
DS(ON)
FIGURE 2. SW Pin Voltage and Inductor Current
- V
needs to be at least 2.5V greater than V
), and the switch rise and fall times. However,
BOOST
SW
BOOST
should not exceed the maximum operating limit
- V
Waveforms of LM3405A
. The voltage across C3, V
SW
is recommended to achieve better ef-
BOOST
SW
30015207
- V
. A large
30015252
SW
, is