ISL97801BOOSTEVALZ【PDF 11/19 页】EVAL BOARD FOR ISL97801

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ISL97801

Theory of Operation

General Description

The ISL97801 is a flexible, highly integrated high-power LED

VIN

FB

driver consisting of a PWM switching controller and

integrated 36V NDMOS power FET. The device can drive up

to 8 series high-power LED's at currents up to 1A at 16V

input or 5 LEDs at current up to 350mA at 2.7V input. The

control loop can be configured as either as a boost or buck

Voltage

Feedback

LEVEL

SHIFT

+

-

GND

regulator with the configuration of the buck/boostn pin,

providing an output voltage above or below the input supply

EL7801

VDC /2

voltage, depending on the number of stacked LED's. The

controller operates from 2.7V to 16V depending on the

numbers of LEDs and current required and can be powered

by a single lithium ion battery, 5V or 12V regulated supplies

or automotive electrical systems. LED current is sensed

through a low value resistor in series with the LED. A

thermistor can be used to implement a thermal protection

scheme to limit the maximum LED temperature to a preset

desirable level.

Switching Regulator

The ISL97801 employs a current mode PWM control

FIGURE 29. FB REFERENCE AUTO SWITCH

Start-up

To maximize external PWM switching speed, the ISL97801

does not include an internal soft-start circuit. When VDC

exceeds the power on reset threshold, switching is delayed

for 1ms (T DELAY ) allowing the output capacitor to charge

through the inductor. If soft-start control is required, a

suitable application circuit is shown in Figure 30.

VBAT

scheme with a nominal switching frequency of 1MHz. This

provides fast transient response and enables the use of low

profile inductors and compact multilayer ceramic capacitors.

Settling time is optimized by the use of a simple control loop

without an error amplifier, relying instead on intrinsic gain

VBAT FAULT

ISL97801 VIN

SWD1

SWD2

10μH

L1

C1

4.7nF

VOUT

COUT

20μF

within the direct summing path. Due to the lower loop gain,

R1

offset must be accounted for when setting up initial LED bias

current. Refer to the applications section of the datasheet for

FB

SWS1 SWS2

R2

2k

100

0.5

RSENSE

further information. Figure 28 shows a block diagram of the

system.

Application Configurations

Operating Modes

The ISL97801 can operate as either a buck or boost

regulator. Hardwire BUCK/BOOSTN to GND for boost mode

or to VDC for buck mode. In buck mode the power NDMOS

drive circuit is "floated" (boot-strapped) allowing the NDMOS

gate to be driven above VIN to fully enhance the power

NDMOS. An internal Schottky diode between VDC (5V) and

VHI reduces external component count. Use a ceramic

capacitor of at least 50nF between VHI and SWS1/2 to

bootstrap VHI.

LED Load Connection

ISL97801 includes an auto-sensing FB level shift circuit that

enables the LED load to be connected to either GND or VIN.

An internal sense circuit monitors the FB pin voltage. When

the level exceeds VDC/2, the feedback reference voltage is

switched from GND to VIN. Refer to the application section

of the datasheet for typical application schematics.

11

FIGURE 30. EXTERNAL SOFT-START CIRCUIT

Light Level Control

Two light control schemes are provided:

1. An external PWM signal via the EN/PWM pin, providing

low frequency PWM dimming.

2. Bias current level adjustment via the LEVEL input or fixed

internal bias.

PWM Dimming

LED color temperature varies with bias current. In

backlighting applications PWM dimming offers better control

of color temperature because current through the LED's is

kept constant. A 5V gate driver (ENL) synchronized to

EN/PWM can be used to control an external N-Ch FET and

disconnect the LED stack during the PWM off period. The

switch prevents discharge of the output capacitor by the LED

load, maintaining a constant bias independent of PWM duty

cycle. Operation at 1kHz PWM rate is shown in Figure 31

and Figure 32. The load disconnect switch improves PWM

dynamic range, linearity and color temperature control. To

FN6428.1

April 2, 2007

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