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ISL97634
Typical Performance Curves (Continued)
V OUT
V OUT
LX
LX
PWM/EN
V IN = 4V
PWM/EN
ILED
R 1 = 4 Ω
L 1 = 22μH
ILED
V IN = 4V
R 1 = 4 Ω
L 1 = 22μH
FIGURE 8. PWM DIMMING AT 1kHz, D = 1% ZOOM IN
FIGURE 9. PWM DIMMING AT 1kHz, D = 99%
V OUT
LX
R 1 = 4 Ω
V IN = 4V
L1 = 22μH
PWM/EN
ILED
FIGURE 10. PWM DIMMING AT 20kHz, D = 50%
I LED = --------------- (EQ. 1)
I LED-AVG = --------------- ? D
Detailed Description
The ISL97634 uses a constant frequency, current mode
control scheme to provide excellent line and load
regulation. There are three OVP thresholds set at 14V, 18V
and 26V respectively. The ISL97634 operates from an input
voltage of 2.4V to 5.5V and ambient temperature from -40°C
to +85°C. The switching frequency is around 1.45MHz and
allows the driver circuit to employ small LC components. The
peak forward current of the LED is set using the R SET resistor.
In the steady state mode, the LED peak current is given by
Equation 1:
V FB
R SET
PWM Dimming
The ISL97634’s PWM/EN pin can be tied permanently to high
for a fixed current operation. On the other hand, the ISL97634
6
can be applied with an external PWM signal to pulse width
modulated output current. It is well understood that the LED
brightness is a linear function of the LED current. In addition,
the average LED current corresponds to the duty cycle “D” of
the PWM signal as shown in Equation 2:
V FB
(EQ. 2)
R SET
As a result, PWM signal provides a means to dim the LED
brightness. PWM dimming offers the best LEDs matching over
DC dimming. It is because the LED peak current operating
point is far away from the knee of the diode I-V curve where
part to part variations are high. The PWM dimming test results
are shown in Figure 7 with two PWM frequencies, 1kHz and
20kHz. The vertical scale parameter FB is proportional to the
current and therefore the brightness.
For the ISL97634, PWM dimming provides linear dimming
adjustment with low frequency signal, such as 1kHz and
below. The applied PWM dimming signal can be up to 32kHz;
FN6264.4
August 27, 2013
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