XRP7604 Datasheet, PDF (9/13 Pages) Exar Corporation – 1A 29V Non-Sync. Buck High Power LED Driver

XRP7604
1A 29V Non-Sync. Buck High Power LED Driver
LED current versus PWM Dimming Duty
100
100Hz
200Hz
500Hz
1kHz
90
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
(1-D) PWM Dimming Duty (%)
Fig. 16: Linearity, LED current vs (1-D) PWM Dimming
Duty Cycle, Vin=12V, Io=0.75A, 2 LED in series
MODULATOR OPERATION AND POWER
SEQUENCING
The XRP7604 has a unique modulator design
which improves the device’s ability to operate
at very high duty cycle. While seamless in
operation as the duty cycle is increasing (input
voltage falling), when the duty cycle is
decreased (input voltage rising), the user will
observe the switching frequency increasing in
distinct fractions of the switching frequency. If
the device is operating at 100% duty cycle, a
unique advantage of using a p-channel pass
device, and then the input voltage is
increased, the frequency will start at 300kHz,
then 600kHz, and then finally 1.2MHz. The
frequency will tend to increase to the next
higher fraction once the duty cycle reaches 75
to 65 percent. This is the normal operation of
the device and should be expected. There is
no impact on the LED current accuracy. If
PWM dimming is being used as the input
voltage is increased, one will see the
frequency increasing when the duty cycle is <
90%. When power is initially applied the
device will begin operating as if the input
voltage is increasing and may start operation
at one of the fractional operating frequencies.
Many users will prefer to have the device start
operating at the nominal operating frequency,
thus it is recommended that Vin be applied
after FB is set at the high state (>1.2V). The
regulator is now in standby and once Vin has
reached steady-state then FB is transitioned
from a high to a low state. The regulator then
starts operating at nominal frequency.
Another benefit of using power sequencing for
power up is that it ensures all internal circuitry
is alive and fully operational before the device
is required to regulate the current through the
LEDs. Since the regulator was “Off” before
power was applied, it is unlikely the LED is
under any type of thermal stress. EXAR does
not recommend using the XRP7604 in
applications where dimming of the LED is
achieved by PWM’ing the actual input power as
is common in automotive dimming
applications.
BUCK OPERATION WITHOUT OUTPUT
CAPACITOR
In order to be able to apply the
aforementioned dimming signal to the LED,
the output filter capacitor that is normally
used with a buck converter has to be removed
from the circuit. Thus the LED current ripple
equals the inductor current ripple. As a rule of
thumb current ripple should be limited to 10%
of ILED. Allowing for a higher current ripple,
up to 30%, while staying within LED
manufacturer ripple guidelines, will reduce
inductance and possibly inductor size.
OVERCURRENT PROGRAMMING
Resistor Rs can be used to program
Overcurrent Protection (OCP). Use the
following equation for calculating the Rs value.
Equ.2:
Rs = ^{0.35V − (1.5 ×1.15 × IOCP × Rds(on))}
33μA
Where Iocp is the programmed overcurrent
and is generally set 50% above nominal
output current, and Rds(on) = 135mohms.
Maximum value of Rs that can be used for
programming OCP is 4k.
INDUCTOR SELECTION
Select the inductor L1 for inductance, Irms
and Isat. Calculate inductance from
© 2009 Exar Corporation
9/13
Rev. 1.0.0