Datasheet ISL97634 (Intersil) - 7

link to page 5 link to page 7 link to page 7 link to page 7 link to page 7 link to page 8 link to page 1 ISL97634 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
VOUT
cause any functional problem. The PWM dimming linear responses in Figure 6 are expanded in Figure 11. 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
LX
from 5% to DC only.
10 7 LEDs PWM/EN 9 VIN = 4V 8 RSET = 4 1kHz VIN = 4V L = 10µH R1 = 4

) 7 ILED V L1 = 22µH (m 6 GE 5 TA
FIGURE 13. PWM DIMMING AT 1kHz WITHOUT USING FBSW
OL 4 V
The FBSW should be used for PWM dimming as illustrated in the
B F 3
“Typical Application Circuit” on page 1. During the PWM off time,
20kHz 2
the FBSW is opened. The LEDs are floating and therefore the output capacitor has no path to discharge. The LED current
1 32kHz
responds accurately with the PWM signal (see Figure 14). The
0
output switches very quickly to the target current with minimal
0 2 4 6 8 10 12
settling ringing and without being modulated by the PWM signal,
DUTY CYCLE (%)
and therefore minimizes any system disturbance. FIGURE 11. DIMMING LINEARITY vs DUTY CYCLES ZOOM IN The low level non-linearity effects at high frequency PWM dimming is also reflected in the efficiency measurements in
VOUT
Figure 12.
90 LX 85 80 75 (%) PWM/EN VIN = 4V 70 R1 = 4

L1 = 22µH 65 EFFICIENCY 3 LEDs 60 ILED VIN = 4V 55 RSET = 4

L = 22µH 50
FIGURE 14. PWM DIMMING AT 1kHz USING FBSW
0 5 10 15 20 25 30 ILED (mA)
Overvoltage Protection FIGURE 12. EFFICIENCY vs PWM DIMMING FREQUENCIES The ISL97634 comes with overvoltage protection. The OVP trip points are at 14V, 18V and 26V for ISL97634IRT14Z, Feedback Disconnect Switch ISL97634IRT18Z and ISL97634IRT26Z respectively. The The ISL97634 functions properly without using the FBSW. maximum LED current and OVP threshold are shown in Table 1. However, the output capacitor will discharge during the PWM off When the device reaches the OVP, the LX stops switching, time resulting in poor dimming linearity at low duty cycles. The disabling the boost circuit until VOUT falls about 7% below the OVP output discharge effect can be seen in Figure 13. Moreover, the threshold. At this point, LX will be allowed to switch again. The OVP output is modulated by the PWM signal that may create event will not cause the device to shutdown. interference to other systems. 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. FN6264 Rev 4.00 Page 7 of 12 August 27, 2013