ADP5350ACPZ-1-R7 datasheet(29/63 Pages) AD

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Data Sheet

ADP5350

Rev. B | Page 29 of 63

BOOST AND WHITE LED DRIVERS

The ADP5350 integrates a powerful 1.5 MHz frequency boost

regulator with programmable LED control. Different LED

configurations, like LEDs in parallel or LEDs in serial, are

supported with careful design. Up to five LED strings are

independently programmable up to 20 mA (typical) in 64 levels.

All LED strings can be individually programmed or combined

into a group to operate as the backlight LEDs or individual LED

current sinks.

A full suite of safety features, including current-limit, overvoltage,

LED open-circuit, and overtemperature protection, allows a

robust and safe design. The integrated soft start limits inrush

currents during start-up and restart attempts.

White LED Driver

White LEDs are common in backlighting the displays of

modern portable devices. White LEDs require a high forward

voltage, VF (typically 3.3 V), before conducting current and

emitting light. Display panels, depending on the size, can be

backlit with multiple white LEDs in series or in parallel. The

LEDs need a common current passing through all of them to

achieve uniform brightness. The LED, however, must be biased

with a voltage greater than the sum of each LED VF voltage

before it can conduct.

The ADP5350 integrates a 1.5 MHz boost regulator to power

the LED bias voltage. If the LED forward voltage plus the

current sink headroom voltage is higher than the battery

voltage, the boost regulator turns on. If the battery voltage is

higher than the sum of the LED forward voltage plus the

required current sink headroom voltage, the boost regulator

operates in passthrough mode.

The ADP5350 uses an integrated negative channel field effect

transistor (NFET) low-side current regulator for accurate

brightness control, with up to five channels of current sink.

The ADP5350 supports setting different LED currents for each

LED string. Any mismatch in the forward voltage of the LEDs

translates directly to lower efficiency, as well as lower accuracy

of the current for the lower voltage LED string.

The boost regulator in the ADP5350 has two operation modes,

LED operation mode and boost standalone operation mode,

which can be selected via the I2C-compatible interface.

LED Operation Mode

When the boost regulator is required to provide a higher output

voltage to the LED bias voltage, the boost regulator must be

configured in LED operation mode by setting BST_MODE = 0

in the BST_CFG register.

In LED operation mode, the boost regulator provides the

adaptive LED bias voltage with adaptive headroom regulation to

optimize the system efficiency against LED forward voltage

variation and aging. The boost regulator is attached to the LED

current source control and, therefore, is automatically activated

by any active LED current source. The EN_BST bit is not

effective in this mode.

Because the LED bias voltage may be coming from the battery

system voltage instead of the boost output voltage (for example,

LED indicators with relatively low forward voltage), those LEDs

can be used in individual current sink channels by using the

battery system voltage as the LED bias voltage. Use the BST_BL

bit in the BST_CFG register to determine whether the bias

voltage for individual current sink channels is coming from the

boost regulator output or from the battery system voltage.

Write 0 to BST_BL to set the boost regulator to provide the bias

voltage for all active LED channels. In this configuration, the

boost regulator provides the adaptive headroom regulation

according to all active LED current sources, including both

backlight and individual current sinks.

Write 1 to BST_BL to set the boost regulator to provide the bias

voltage only for the active LED backlight channels, excluding

individual current sink. The bias voltage for an individual LED

sink can be from the battery system voltage or from some other

fixed rail; therefore, the headroom status in individual LED

sinks does not affect the boost output regulation. BST_BL must

be set to 1 when the indicator LED is connected to the battery

system voltage instead of the boost output voltage; otherwise,

the boost voltage may risk an overvoltage. The adaptive headroom

control in the boost regulator may include individual LED

channels whose bias voltage is not coming from the boost

regulator.

The boost feedback pin (FB4 pin) is tied to ground in LED

operation mode.

Boost Standalone Operation Mode

When the boost regulator is used to provide the fixed output

voltage for other system uses, including organic light emitting

diode (OLED) backlight, audio system, or other auxiliary

circuitries, the boost regulator must be configured in standalone

operation mode by setting BST_MODE = 1 in the BST_CFG

below 800 mW when the boost peak current is set to 600 mA.

In standalone operation mode, the boost regulator provides the

adjustable output voltage, VOUT4, configured by the external

resistor divider.

VOUT4 = VFB4 × (RFB1 + RFB2)/RFB2

The activation status of the boost regulator is determined by the

EN_BST bit in the BST_CFG register. In boost standalone

operation mode, all LED functions are turned off and not

allowed.

In standalone operation mode, the boost feedback pin (FB4 pin)

must be tied to the boost output through an external resistor

divider.

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