AOZ1935QI【PDF 11/15 页】IC LED DRVR WHITE BCKLGT 16QFN

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AOZ1935

Detailed Description

The AOZ1934/5 is a unique current regulated step-up

(boost) converter. Two (or three for AOZ1935) current

regulating devices are integrated to drive two or three

strings of LEDs. Unused channels need to be connected

to ground (GND).

The voltage step-up is accomplished by a boost topology,

using an inductor-based DC-DC switching converter, in

which the inductor serves as an energy storage device in

the system. By integrating a high voltage optimized

MOSFET, the AOZ1934/5 internal switching frequency is

800kHz while still maintaining high power efficiency.

Unlike a traditional DC-DC boost converter with a fixed

output voltage, the AOZ1934/5 dynamically changes its

output voltage regulation parameters depending on the

load. The use of unique control schemes maintains

accurate current regulation in each current sink while

leaving the output voltage at a minimum, increasing the

overall conversion efficiency. The internal step-up

converter dynamically controls the voltage at the output

high enough to drive the LED string with the highest total

forward voltage. The wide input voltage range, from 2.7V

to 25V, allows maximum system flexibility; battery

powered devices using 1-, 2-, 3- or 4-cell Li Ion packs

can share the same power architecture. A low power

2.7V to 5.5V input to supply the IC bias circuitry is

required at AVCC pin.

LED brightness control is based on the popular PWM

dimming scheme via the PWM pin. The EN pin provides

ON/OFF control of the IC. By asserting a logic high on

the EN pin, the converter is enabled and will regulate

each of the current sinks at the factory programmed

current setting.

Application Information

Power Up Sequence

It’s recommended to power up Vin, AVCC, and PWM

dimming in sequence, and EN pin needs to ramp up

simultaneously or later than AVCC, in order to avoid

latch-off, or mis-operation of device. We do not assume

any liability or responsibility for any loss or damage

arising from misuse or inappropriate use of the product.

Current Dimming Using PWM Pin

AOZ1934/5 allows maximum flexibility with a PWM signal

control input which can be used for dimming. The IC

must remain Enabled at EN pin during PWM dimming,

which means the step-up converter is active to reduce

the response time. Combining this with the fast response

of the step-up converter, each LED channel can typically

recover from no current within 1μs. This allows very short

PWM on-time or a duty-cycle with fine resolution.

Assume a 50μs minimum on-time for a 100Hz PWM

signal, a 200:1 ratio can be achieved by PWM.

The minimum PWM on-time of 1 ? s is required in

AOZ1934/5’s application, this means minimum PWM

duty cycle of 2% for 20kHz dimming frequency and

0.02% for 200Hz dimming frequency. The amplitude of

PWM signal should not be higher than AVCC, otherwise

test mode will be triggered.

Fault Protection (LED Open or Short)

Each current sink is protected against LED short or

open conditions. If an LED short circuit condition

arises, the current sink continues to regulate until

V SINK > V SOV (6V). When any sink node voltage goes

above V SOV for more than 80ms (while PWM dimming

signal remains high), that channel’s current sink will be

turned off, and other channels will still work if they don’t

trigger this fault condition. For example, if one or more

LEDs on a channel are shorted, that channel’s sink

voltage will increase. If the voltage goes above 6V for

more than 80ms, the Current Sink Fault Protection will be

triggered and only this faulty string will be disabled by

shutting off this current sink. All other channels will

continue normal operation if they don’t have a fault

condition. When PWM dimming control is used, the 80ms

fault delay is accumulated only when PWM signal is high;

the PWM low time is not counted into the fault delay, so

the delay time will be 80ms / D PWM (D PWM is the duty

cycle of the PWM signal).

In case of an LED failing open, the current sink voltage of

the failed string will go close to ground and dominate the

boost converter control loop. As a result the output

voltage will move up to the over-voltage threshold, set

internally to 40V or by the external resistor divider,

whichever is lower. Once the over-voltage incident is

flagged internally, the faulty channel(s) will be disabled.

Then the output voltage of the boost converter will go

back to normal level. During the entire process, the rest

of the LED strings (healthy LED strings) would continue

normal operation.

The fault conditions are reset when the IC is powered

down and up again.

Output Over-Voltage Protection (OVP)

AOZ1934/5 has built in over-voltage protection function

with 40V threshold at VOUT pin. This level is suitable for

applications with ten series LEDs per string. However, for

applications with fewer series LEDs, the user can choose

Rev. 1.2 June 2013

www.aosmd.com

Page 11 of 15

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