ISL97632IRT14ZT 查看數據表（PDF） - Intersil

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ISL97632IRT14ZT

LED Driver with 1-Wire Dimming

Intersil 

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----F---B----(---S-----)

RSET

=

9----.--8---m------V-----+-----5----.-6----8---m------V-----×-----S--

RSET

(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

=

IL

A

V

G

+

1--

2

⋅

ΔIL

(EQ. 3)

ΔIL

=

V-----I--N-----⋅---(--V-----O----U----T-----–----V-----I--N----)

L ⋅ VOUT ⋅ fOSC

Where:

(EQ. 4)

• ΔIL 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.

6

FN9239.3

March 22, 2010

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