ISL97634

7

FN6264.4

August 27, 2013

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 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

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 12.

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 13. 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 the

“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 14). 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 LED current and OVP threshold are shown in Table 1.

When the device reaches the OVP, the LX stops switching,

threshold. At this point, LX will be allowed to switch again. The OVP

event will not cause the device to shutdown.

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 11. DIMMING LINEARITY vs DUTY CYCLES ZOOM IN

0

1

2

3

4

5

6

7

8

9

10

02468

10

12

DUTY CYCLE (%)

7 LEDs

RSET = 4

Ω

L = 10µH

20kHz

32kHz

1kHz

FIGURE 12. EFFICIENCY vs PWM DIMMING FREQUENCIES

50

55

60

65

70

75

80

85

90

0

5

10

15

20

25

30

ILED (mA)

RSET = 4

Ω

L = 22µH

3 LEDs

ILED

PWM/EN

LX

R1 = 4

Ω

L1 = 22µH

FIGURE 13. PWM DIMMING AT 1kHz WITHOUT USING FBSW

FIGURE 14. PWM DIMMING AT 1kHz USING FBSW

ILED

PWM/EN

LX

R1 = 4

Ω

L1 = 22µH