ISL97634 데이터 시트보기 (PDF) - Intersil

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ISL97634

White LED Driver with Wide PWM Dimming Range

Intersil 

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

R1 = 4Ω

VIN = 4V

L1 = 22µH

PWM/EN

LX

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 7 LEDs (3.5V/20mA type) 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 Equation1:

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 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 shown in

Equation 2:

IL E D -AVG

=

---V----F---B----

RSET

⋅

D

(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

5

FN6264.3

March 7, 2008

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