ISL97634(2007) データシートの表示（PDF） - Intersil
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ISL97634 Datasheet PDF : 10 Pages
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VOUT
LX
PWM/EN
ILED
ISL97634
VIN = 4V
R1 = 4Ω
L1 = 22µH
VOUT
LX
PWM/EN
ILED
VIN = 4V
R1 = 4Ω
L1 = 22µH
FIGURE 7. PWM DIMMING AT 1kHz, D = 1% ZOOM IN
FIGURE 8. PWM DIMMING AT 1kHz, D = 99%
VOUT
LX
PWM/EN
R1 = 4Ω
VIN = 4V
L1 = 22µH
ILED
FIGURE 9. PWM DIMMING AT 20kHz, D = 50%
Detailed Description
The ISL97634 uses a constant frequency, current mode
control scheme to provide excellent line and load
regulation. There are three OVP models for driving 3, 4,
and 6 LEDs and their OVP thresholds are 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 RSET resistor. In the steady state mode, the LED
peak current is given by the following equation:
ILED
=
---V----F----B---
RSET
(EQ. 1)
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 can be applied with an external PWM signal to
pulse width modulated the 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:
ILED – AVG
=
---V----F---B---- ⋅ D
RSET
(EQ. 2)
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 6 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
5
FN6264.2
April 10, 2007
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