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ISL97634 Datasheet(PDF) 6 Page - Intersil Corporation

Part No. ISL97634
Description  White LED Driver with Wide PWM Dimming Range
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Maker  INTERSIL [Intersil Corporation]
Homepage  http://www.intersil.com/cda/home
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ISL97634 Datasheet(HTML) 6 Page - Intersil Corporation

 
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6
FN6264.2
April 10, 2007
; however, the dimming linearity is compromised at low duty
cycles as their durations are too short for the ISL97634’s
control loop to respond properly. This non ideality behavior
does not cause any functional problem. The PWM dimming
linear responses in Figure 5 are expanded in Figure 10. At
1kHz PWM dimming, the duty cycle can virtually vary from
below 1% to DC. On the other hand, at 20kHz PWM
dimming, the linearity range is from 5% to DC only.
The low level non-linearity effects at high frequency PWM
dimming is also reflected in the efficiency measurements in
Figure 11.
Feedback Disconnect Switch
The ISL97634 functions properly without using the FBSW.
However, the output capacitor will discharge during the PWM
off time resulting in poor dimming linearity at low duty cycles.
The output discharge effect can be seen in Figure 12.
Moreover, the output is modulated by the PWM signal that
may create interference to other systems.
.
The FBSW should be used for PWM dimming as illustrated
in “Typical Application Circuit” on page 1. During the PWM
off time, the FBSW is opened. The LEDs are floating and
therefore the output capacitor has no path to discharge. The
LED current responds accurately with the PWM signal (see
Figure 13). The output switches very quickly to the target
current with minimal settling ringing and without being
modulated by the PWM signal and therefore minimizes any
system disturbance.
Overvoltage Protection
The ISL97634 comes with overvoltage protection. The OVP
trip points are at 14V, 18V, and 26V for ISL97634IRT14Z,
ISL97634IRT18Z, and ISL97634IRT26Z respectively. The
maximum numbers of LEDs and OVP threshold are shown in
Table 1. When the device reaches the OVP, the LX stops
switching, disabling the boost circuit until VOUT falls about 7%
below the OVP threshold. At this point, LX will be allowed to
switch again. The OVP event will not cause the device to
shutdown.
There are three OVP options so that the 3 LEDs application
should use the 14V OVP device and the 6 LEDs application
should use the 26V OVP device. An output capacitor that is
only rated for the required voltage range can therefore be
used, which will optimize the component costs in some cases.
FIGURE 10. DIMMING LINEARITY vs DUTY CYCLES ZOOM IN
0
1
2
3
4
5
6
7
8
9
10
02468
10
12
DUTY CYCLE (%)
6 LEDs
VIN = 4V
RSET = 4
Ω
L = 10µH
20kHz
32kHz
1kHz
FIGURE 11. EFFICIENCY vs PWM DIMMING FREQUENCIES
50
55
60
65
70
75
80
85
90
0
5
10
15
20
25
30
ILED (mA)
VIN = 4V
RSET = 4
Ω
L = 22µH
3 LEDs
ILED
PWM/EN
LX
VOUT
VIN = 4V
R1 = 4
Ω
L1 = 22µH
FIGURE 12. PWM DIMMING AT 1kHz WITHOUT USING FBSW
FIGURE 13. PWM DIMMING AT 1kHz USING FBSW
ILED
PWM/EN
LX
VOUT
VIN = 4V
R1 = 4
Ω
L1 = 22µH
ISL97634


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