LM3519 Datasheet, PDF (4/20 Pages) National Semiconductor (TI) – High Frequency Boost White LED Driver with High-Speed PWM Brightness Control

LM3519
SNVS394A – AUGUST 2005 – REVISED SEPTEMBER 2005
Block Diagram
Vout
3
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Sw
5
Vin
6
OVER
VOLTAGE
PROTECTION
I_REF
CURRENT
LIMIT
En
V_REF
1
ERROR
AMPLIFIER
+
R
R
N3
Vin
-
ON-TIME
GENERATOR
S ^DRIVER Q
LOGIC
N1
Vin
N2
4
2
LED_rtn
Gnd
Figure 3. Block Diagram
Circuit Description
The LM3519 is a step-up converter for white LED applications that uses a unique and proprietary pulse
frequency modulation (PFM) architecture to optimize high efficiency at high frequency operation. Unlike most
PFM architecture implementations, the LM3519’s unique architectural implementation results in non-pulse
skipping variable frequency operation. The regulator is forced to operate at the edge of Continous Conduction
Mode (CCM). The error amplifier will set the end of the on-time (I_PEAK of inductor) based on the load (LEDs)
current. During this operation, the inductor current ramps up and reaches a peak current at end of the on-time. At
this point, the internal power switch is turned off until the inductor current reaches zero, and the cycle repeats
again. The switching frequency is set based on the charge (on-time) and discharge(off-time) of the inductor
current. The frequency can range between 2MHz to 8MHz over the operating input range.
The LM3519 operation can be best understood through an examination of the block diagram in Figure 3. When
LED current is out of regulation, the LED_rtn voltage falls below or rises above the internal reference voltage
(VREF). The error amplifier will output a signal to increase or decrease the proper on-time duration of N1 power
FET. This correction allows the inductor's stored energy to increase or decrease to a sufficient level that when
transferred to the load will bring the LED_rtn current back into regulation.
During steady-state operation for a typical switching cycle, the oscillator sets the driver logic and turns on N1
power device. N1 conducts current through the inductor and reverse biases the external diode. The LED current
is supplied by the output capacitor when N1 is conducting. Once N1 on-time period is concluded, the internal
power device is turned off and the external diode is forward baised. The inductor current then flows through the
diode to the LED load to replenish the output capacitor and keep the LED current regulated at the trimmed
target.
4
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