Chip information – Rainbow Electronics MAX16825 User Manual
Page 12
MAX16824/MAX16825
Another method of pulse dimming the outputs is to hold
LE high and drive OE with a PWM signal. Because the
input bits are always latched, the serial input constantly
refreshes the register. The control bits must be carefully
selected to dim the outputs properly.
A third method is to hold both LE and OE in the
enabled state. This allows the data bits to directly con-
trol the output channels, and hence, pulse dim the out-
put current. Make sure that the clock frequency does
not exceed the maximum rate at which the device can
change the state of the output channels.
Power Dissipation
The power dissipation (P
D
) of the MAX16824/
MAX16825 is determined from the following equation:
where:
V
IN
= supply voltage
V
INL
= supply Voltage to the LED strings
I
IN
= supply current
V
LED_
= total forward voltage for one LED string
I
LED_
= LED current
V
CS
= 200mV drop across R
CS_
DUTY = PWM_ duty cycle
The worst-case power dissipation occurs when the
drop across each internal MOSFET is at its maximum
with all three channels delivering the maximum allow-
able output current. The maximum drop across the
internal MOSFETs is determined by:
V
INL
- V
LED_
- V
CS_
when V
LED_
is at its minimum.
Higher ambient temperature increases the thermal stress
even further due to the reduction in voltage drop across
the LEDs. The MAX16824/MAX16825 thermal specifica-
tions are given according to the JEDEC-51 guidelines.
Good mechanical/thermal design practices must be
applied to help maintain the device junction temperature
below the absolute maximum ratings at all times.
Chip Information
PROCESS: BiCMOS-DMOS
P
V
I
V
V
V
I
DUTY
V
V
V
I
DUTY
V
V
V
I
DUTY
D
IN
IN
INL
LED
CS
LED
INL
LED
CS
LED
INL
LED
CS
LED
=
Ч
+
−
−
Ч
Ч
+
−
−
Ч
Ч
+
−
−
Ч
Ч
(
) (
) (
)
(
) (
)
(
) (
)
1
1
1
2
2
2
3
3
3
1
2
3
High-Voltage, Three-Channel Linear
High-Brightness LED Drivers
12
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