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? 2 . 2 ? ? f PWM ? ? 100 %
? I OUT
? 2 . 2 ? ? 3 k ? ? 100 ? ? 6 . 6 %
? I OUT
PWM Dimming
PWM dimming can be implemented by toggling the
enable (EN) pin (pin 4). The recommended PWM
frequency range is 100Hz to 3kHz. For example, if the
rise time is 2.2μs, the actual duty cycle applied internally
to the output MOSFETs is slightly less than the duty
cycle of the signal applied on the enable pin. This leads
to a slight non-linearity in the measured LED current.
That error is:
? ? EQ. 4
I OUT _ SET D PWM
For example, at a PWM frequency of 3kHz, with an
applied duty cycle of 10%, the typical error is:
? ? EQ. 5
I OUT _ SET 0 . 1
So, if R SET is 5.225k ? , the theoretically expected LED
current, with a PWM duty cycle of 10%, is 2.5mA.
However, the actual (measured) LED current is less by
6.6%. It is (1-0.066) multiplied by 2.5mA, which is
2.335mA. In this way, the actual LED current for any
PWM duty cycle and frequency can be estimated.
Input Rail Dimming
The LEDs can also be dimmed by modulating the input
supply rail. See Figure 15, PWM Dimming By VIN Pin,
under Typical Characteristics. A maximum frequency of
1KHz is recommended.
Power Dissipation
At an ambient temperature (T A ), the power dissipation
(P D ) and the junction temperature (T J ) are related to
each other as described in the following equation:
This should be solved for T J_MAX and the result verified
as less than the over-temperature shutdown threshold of
150oC (typical). An additional 25oC margin is
recommended to account for tolerances on the
shutdown threshold; T J_MAX should not exceed 125oC.
The ? JA is dependent on the surrounding PCB layout
and can be around 300oC/W for an SC-70 package.
This can be improved by providing a heat sink of
surrounding copper ground on the PCB. The addition of
backside copper with vias, stiffeners, and other
enhancements can reduce this value. The heat
contributed by the dissipation of other devices located
nearby must be included in design considerations. Once
the limiting parameters in these two relationships have
been determined, the design can be modified to ensure
that the device remains within specified operating
conditions. If overload conditions are not considered, it
is possible for the device to enter a thermal cycling loop,
in which the circuit enters a shutdown condition, cools,
re-enables, and again overheats and shuts down
repeatedly due to an unmanaged fault condition.
LED Selection
The FAN5640 is designed to drive 2-4 LEDs or a higher
number of monochrome LEDs. The maximum number of
LEDs per channel can be calculated as a function of V IN
and the sum of the forward voltage of each LED at the
maximum specified current. The minimum number of
LEDs driven by FAN5640 is the result of calculating the
maximum power dissipated by the IC in the given
operating conditions. The forward voltage of LEDs
depends upon type of LEDs and the manufacturer. In
terms of maximum number of LEDs and LED current,
refer to the Dropout Voltage vs. LED Current graph in
the Typical Characteristics ( see Figure 8 ).
Manufacturer Part Website
T J ? T A ? P D ? ? JA
where:
P D ? ( V IN ? V O ) ? I OUT _ Total ? V IN ? I Q ?
V RSET
R EXT
EQ. 6
? ? V IN ? V RSET ?
HARVATEK
NICHIA
HT-T169TW
NSSW1087
www.harvatek.com
www.nichia.com
and
I OUT_Total = I OUT1 + I OUT2 .
The quiescent current (I Q ) can be found in the Electrical
Characteristics section. The junction-to-ambient thermal
resistance ( ? JA ) puts a limit on V O_MAX , I OUT_MAX , and the
maximum dropout (V IN -V O ) MAX . This affects the number
of LEDs used, the current used to drive them, and so
on. Ensure that thermal shutdown does not occur. The
formula that correlates all these variables is:
( V IN ? V O ) MAX ?
T J _ MAX ? T A _ MAX
? JA ? I OUT _ Total
EQ. 7
? 2006 Fairchild Semiconductor Corporation
FAN5640 ? Rev. 1.0.2
9
www.fairchildsemi.com
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