LM3528 datasheet(13/34 Pages) TI1

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IOUT_MAX =

VOUT

(IPEAK

'IL) u K u VIN

'IL =

VIN u (VOUT

VIN)

2 u fSW u L u VOUT

where

LM3528

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SNVS513B – AUGUST 2008 – REVISED MAY 2013

Adaptive Regulation

When biasing dual white led strings (White LED mode) the LM3528 maximizes efficiency by adaptively regulating

the output voltage. In this configuration the 500mV reference is connected to the non-inverting input of the error

amplifier via mux S2 (see Figure 37). The lowest of either VMAIN or VSUB/FB is then applied to the inverting input of

the error amplifier via mux S1. This ensures that VMAIN and VSUB/FB are at least 500mV, thus providing enough

voltage headroom at the input to the current sinks for proper current regulation.

In the instance when there are unequal numbers of LEDs or unequal currents from string to string, the string with

the highest voltage will be the regulation point.

Unison/Non-Unison Mode

Within White LED mode there are two separate modes of operation, Unison and Non-Unison. Non-Unison mode

provides for independent current regulation, while Unison mode gives up independent regulation for more

accurate matching between LED strings. When in Non-Unison mode the LED currents IMAIN and ISUB/FB are

independently controlled via registers BMAIN and BSUB respectively (see Brightness Registers (BMAIN and

BSUB) section). When in Unison mode BSUB is disabled and both IMAIN and ISUB/FB are controlled via BMAIN

only.

Start-Up

The LM3528 features an internal soft-start, preventing large inrush currents during start-up that can cause

excessive voltage ripple on the input. For the typical application circuits when the device is brought out of

shutdown the average input current ramps from zero to 450mA in approximately 1.2ms. See Start Up Plots in the

Typical Performance Characteristics.

OLED Mode

When the LM3528 is configured for a single White LED bias + OLED display bias (OLED mode), the non-

inverting input of the error amplifier is connected to the internal 1.21V reference via MUX S2. MUX S1 switches

SUB/FB to the inverting input of the error amplifier while disconnecting the internal current sink at SUB/FB. The

voltage at MAIN is not regulated in OLED mode so when the application requires white LED + OLED panel

biasing, ensure that at least 300mV of headroom is maintained at MAIN to ensure proper regulation of IMAIN. (see

the Typical Performance Characteristics for a plot of ILED vs Current Source Headroom Voltage)

Peak Current Limit

The LM3528’s boost converter has a peak current limit for the internal power switch of 770mA typical (650mA

minimum). When the peak switch current reaches the current limit the duty cycle is terminated resulting in a limit

on the maximum output current and thus the maximum output power the LM3528 can deliver. Calculate the

maximum LED current as a function of VIN, VOUT, L and IPEAK as:

(1)

ƒSW = 1.27MHz. Typical values for efficiency and IPEAK can be found in the efficiency and IPEAK curves in the

Typical Performance Characteristics.

Over Voltage Protection

The LM3528's output voltage (VOUT) is limited on the high end by the Output Over-Voltage Protection Threshold

(VOVP) of 21.2V (min). In White LED mode during output open circuit conditions the output voltage will rise to the

over voltage protection threshold. When this happens the controller will stop switching causing VOUT to droop.

When the output voltage drops below 19.7V (min) the device will resume switching. If the device remains in an

over voltage condition the LM3528 will repeat the cycle causing the output to cycle between the high and low

OVP thresholds. See waveform for OVP condition in the Typical Performance Characteristics.

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