ISL97632 データシートの表示（PDF） - Renesas Electronics

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ISL97632

LED Driver with 1-Wire Dimming

Renesas Electronics 

ISL97632

Detailed Description

The ISL97632 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 ISL97632 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 forward current of the LED is set using the

RSET resistor. In the steady state mode, the LED current is

given by Equation 1:

ILEDS = -V--R--F---SB----E---ST----- = 9----.--8---m------V-----+-R----5-S---.E-6---T-8---m------V----------S--

(EQ. 1)

where S is the 5-bit Serial Interface Setting or Digital code from

0 to 31 programmed in the XSD single-wire interface. The

default setting is 0 and the VFB is at minimum.

Dimming Control

The ISL97632 powers up to provide minimium current. By

programming the digital code with the Intersil’s 1-Wire XSD

interface as shown in Figure 1, the current can be changed

linearly with the digital code from 0 to 31. Figure 6 shows LED

current versus the programming codes.

Overvoltage Protection

The ISL97632 comes with overvoltage protection. The OVP

trip points are at 14V, 18V, and 26V for ISL97632IRT14Z,

ISL97632IRT18Z, and ISL97632IRT26Z 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.

TABLE 1.

PART NO.

ISL97632IRT14Z

ISL97632IRT18Z

ISL97632IRT26Z

OVP

14V

18V

26V

MAX NO. OF

LEDS

MAX ILED

3

70mA

4

50mA

7

30mA

There are three OVP options. The 3 LEDs application should

use the 14V OVP device. The 7 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.

Shut-Down

An active high EN pin is normally on but this pin can be used

as a shutdown power saving function or zero brightness

setting. When taken low the EN pin places the ISL97632 into

power down mode down where the supply current is reduced

to less than 1µA. The EN pin cannot be used as PWM input, as

the part resets to 0 whenever EN is low. To resume previous

setting, the device needs to be reprogrammed.

Output Disconnect

The ISL97632 features a FBSW feedback disconnect switch

that can be used in between the LED and RSET for an optional

short-circuit protection. For example, the user may build an

external short circuit detection to monitor the VOUT. If the

VOUT goes low due to one or more LEDs which are shorted,

the circuit can release the EN and FBSW switch to disconnect

the LEDs.

Components Selection

The input capacitance is typically 0.22µF to 4.7µF. The output

capacitor should be in the range of 0.22µF to 1µF. X5R or X7R

type of ceramic capacitors of the appropriate voltage rating are

recommended.

When choosing an inductor, make sure the average and peak

current ratings are adequate by using Equations 2, 3 and 4

(80% efficiency assumed):

ILAVG = -I-L----E0---.-D-8-------V-V---O-I--N--U----T-

(EQ. 2)

ILPK = ILAVG + 12--  IL

(EQ. 3)

IL = V-----IL--N------V----OV----U-O----TU----T---f--–O----V-S----IC--N----

Where:

(EQ. 4)

• IL is the peak-to-peak inductor current ripple in Amps

• L is the inductance in H.

• fOSC is the switching frequency, typically 1.45MHz

The ISL97632 supports a wide range of inductance values

(10µH~82µH). For lower inductor values or lighter loads, the

boost inductor current may become discontinuous. For high

boost inductor values, the boost inductor current will be in

continuous mode.

In addition to the inductor value and switching frequency, the

input voltage, the number of LEDs and the LED current also

affect whether the converter operates in continuous conduction

or discontinuous conduction mode. Both operating modes are

allowed and normal. The discontinuos conduction mode yields

lower efficiency due to higher peak current.

Compensation

The product of the output capacitor and the load create a pole

while the inductor creates a right half plane zero. Both

attributes degrade the phase margin but the ISL97632 has an

internal compensation network that ensures the device

operates reliabily under the specified conditions. The internal

compensation and the highly integrated functions of the

ISL97632 make it a design friendly device to be used in high

volume high reliability applications.

FN9239 Rev 4.00

March 22, 2010

Page 6 of 10

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