XRP7604 Exar Corporation, XRP7604 Datasheet - Page 9

• Manufacturer Part Number: XRP7604
• Description: 1a 29v Non-sync. Buck High Power Led Driver
• Manufacturer: Exar Corporation

M
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
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,
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
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
© 2009 Exar Corporation
Fig. 16: Linearity, LED current vs (1-D) PWM Dimming
ODULATOR OPERATION AND POWER
Duty Cycle, Vin=12V, Io=0.75A, 2 LED in series
100
90
80
70
60
50
40
30
20
10
0
0
falling),
and
is
LED current versus PWM Dimming Duty
10
increased,
100Hz
20
then
(1-D) PWM Dimming Duty (%)
30
when
200Hz
40
the
50
one
the
input
60
500Hz
duty
will
70
voltage
80
1
1
1kHz
see
cycle
A
A
90
2
2
9
9
the
100
V
V
is
is
N
N
9/13
o
o
n
n
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
applications where dimming of the LED is
achieved by PWM’ing the actual input power as
is
applications.
B
CAPACITOR
In
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
manufacturer ripple guidelines, will reduce
inductance and possibly inductor size.
O
Resistor
Overcurrent
following equation for calculating the Rs value.
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.
I
Select the inductor L1 for inductance, Irms
and Isat. Calculate inductance from
-
-
NDUCTOR SELECTION
S
S
UCK OPERATION WITHOUT OUTPUT
VERCURRENT PROGRAMMING
y
y
Rs
n
n
order
to
common
recommend
c
c
=
.
.
. 0
B
B
30%,
Rs
35
u
u
c
c
V
to
k
k
Protection
−
can
H
H
1 (
while
in
be
i
i
5 .
g
g
using
Equ.2:
×
h
h
be
. 1
automotive
33
P
able
P
15
staying
o
o
μ
×
used
w
w
A
(OCP).
the
I
e
OCP
e
to
r
r
×
X
L
X
L
XRP7604
to
E
E
Rds
within
R
R
apply
D
D
P
P
Use
Rev. 1.0.0
(
dimming
program
D
D
on
7
7
r
r
6
6
))
i
i
v
v
0
0
LED
the
the
e
e
4
4
in
r
r