ISL97631 데이터 시트보기 (PDF) - Renesas Electronics
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ISL97631 Datasheet PDF : 8 Pages
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ISL97631
The magnitude of the PWM signal should be higher than the
minimum ENAB voltage high. The bench PWM dimming test
results are shown in Figure 8. In the test, two PWM
frequencies 400Hz and 1kHz are chosen to compare the linear
dimming range. It is clear that there is a wider linear dimming
range for the lower PWM frequency than for the higher one,
due to the self discharge of the output capacitor through the
LEDs during the low ENAB periods. To achieve a better
linearity with high frequencies an NMOS FET can be placed
between the FB pin and the LED stack, with its gate driven by
the same signal as ENAB. This acts to prevent self discharge
of the output capacitor during the off periods. In the PWM
dimming test, the output capacitor is 0.22µF.
20
18
16
14
12
10
8
6
4
2
0
0
1kHz
400Hz
20
40
60
80
100
DUTY-CYCLE (%)
FIGURE 8. PWM DIMMING LINEAR RANGE (FOR 400Hz AND
1kHz PWM FREQUENCIES CONDITION,
COUT = 0.22µF)
ANALOG DIMMING
The second dimming method applies a variable DC voltage
(VDim) at FB pin (see Figure 9) to adjust the LED current. As
the DC dimming signal voltage increases above VFB, the
voltages drop on R1 and R2 increase and the voltage drop on
RSET decreases. Thus, the LED current decreases.
ILED = V-----F---B-----------R----1-R----+-2----R---R-2----S---–E----T-V----D----i--m--------R-----1-
(EQ. 3)
The DC dimming signal voltage can be a variable DC voltage
or a DC voltage generated by filtering a high frequency PWM
control signal.
As brightness is directly proportional to LED currents, VDim
may be calculated for any desired “relative brightness” (F)
using Equation 4.
VDim
=
RR-----21-
VFB
1
+
RR-----12-
–
F
(EQ. 4)
Where F = ILED (dimmed)/ILED (undimmed).
These equations are valid for values of R1 and R2 such that
both R1>>RSET and R2>>RSET.
VIN
2.7V~5.5V
C1
1µF
OFF/ON
L1
22µH
VIN
LX
VOUT
ISL97631
ENAB FB
GND
R2
LEDs
C2
R1
0.22µF
3.3k
RSET
4.75
VDim
FIGURE 9. ANALOG DIMMING CONTROL APPLICATION
CIRCUIT
The analog dimming circuit can be tailored to a desired relative
brightness for different VDim ranges using Equation 5.
R2 = -----V-----D--V--i-m-F----B_---m----a---x--1---–--–--V--F--F--m--B---i-n--------R----1----
(EQ. 5)
Where VDim_max is the maximum VDim voltage and Fmin is the
minimum relative brightness (i.e., the brightness with
VDim_max applied).
i.e., VDim_max = 5V, Fmin = 10% (i.e., 0.1), R2 = 189k
i.e., VDim_max = 1V, Fmin = 10% (i.e., 0.1), R2 = 35k
Open-Voltage Protection
In some applications, it is possible that the output is
opened, e.g. when the LEDs are disconnected from the
circuit or the LEDs fail. In this case the feedback voltage will
be zero. The ISL97631 will then switch to a high duty cycle
resulting in a high output voltage, which may cause the LX pin
voltage to exceed its maximum 27V rating. To implement
overvoltage protection, a zener diode Dz and a resistor R1
can be used at the output and FB pin to limit the voltage on
the LX pin as shown in Figure 10. It is clear that as the zener is
turned on, due to the overvoltage, the zener diode’s current will
set up a voltage on R1 and RSET and this voltage is applied on
FB pin as the feedback node. This feedback will prevent the
output from reaching the overvoltage condition. In the
overvoltage protection circuit design, the zener voltage should
be larger than the maximum forward voltage of the LED
string.
FN7370 Rev 1.00
December 21, 2005
Page 5 of 8
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