FAN5640S7X Datasheet, PDF (9/11 Pages) Fairchild Semiconductor – FAN5640 Dual High-Side Constant Current Source for High-Voltage Keypad LED Illumination

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:
  I_OUT
I_{OUT _ SET}

2.2  f_PWM
D_PWM
 100%
EQ. 4
For example, at a PWM frequency of 3kHz, with an
applied duty cycle of 10%, the typical error is:
I_OUT
I_{OUT _ SET}


2.2 
0.1
3k


100

6.6%
EQ. 5
So, if R_SET is 5.225kΩ, 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:
T_J  T_A  P_D  _JA
EQ. 6
where:
  P_D
 (V_IN
 V_O )  I _{OUT _ Total}
 V_IN  I_Q

V_RSET
R_EXT

V_IN
 V_RSET
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
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
HARVATEK
NICHIA
Part
HT-T169TW
NSSW1087
Website
www.harvatek.com
www.nichia.com
© 2006 Fairchild Semiconductor Corporation
FAN5640 • Rev. 1.0.2
9
www.fairchildsemi.com