ADD5207 Ver la hoja de datos (PDF) - Analog Devices

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ADD5207

Four-String, White LED Driver for LCD Backlight Applications

Analog Devices 

ADD5207

Data Sheet

the output voltage. When the OVP pin voltage reaches the OVP

rising threshold, the boost converter stops switching, which causes

the output voltage to drop. When the OVP pin voltage drops below

the OVP falling threshold, the boot converter begins switching

again, causing the output to rise. There is about 0.8 V hysteresis

between the rising and falling thresholds. The OVP level is fixed

at 39 V (typical).

Open-Load Protection (OLP)

The ADD5207 contains a headroom control circuit to minimize

power loss at each current source. Therefore, the minimum

feedback voltage is achieved by regulating the output voltage of

the boost converter. If any LED string is open circuit during

normal operation, the current source headroom voltage (VHR) is

pulled to GND. In this condition, OLP is activated if VHR is less

than 150 mV until the boost converter output voltage rises up to

the OVP level.

Undervoltage Lockout (UVLO)

An undervoltage lockout circuit is included with built-in hysteresis.

The ADD5207 turns on when VIN rises above 5.0 V (typical) and

shuts down when VIN falls below 4.2 V (typical).

Thermal Protection

Thermal overload protection prevents excessive power dissipa-

tion from overheating and damaging the ADD5207. When the

junction temperature (TJ) exceeds 160°C, a thermal sensor

immediately activates the fault protection, which shuts down

the device and allows it to cool. The device self-starts when the

junction temperature (TJ) of the die falls below 130°C.

EXTERNAL COMPONENT SELECTION GUIDE

Inductor Selection

The inductor is an integral part of the step-up converter. It stores

energy during the switch’s on time and transfers that energy to

the output through the output diode during the switch’s off

time. An inductor in the range of 4.7 µH to 22 µH is

recommended. In general, lower inductance values result in

higher saturation current and lower series resistance for a given

physical size. However, lower inductance results in higher peak

current, which can lead to reduced efficiency and greater input

and/or output ripple and noise. Peak-to-peak inductor ripple

current at close to 30% of the maximum dc input current

typically yields an optimal compromise.

The input (VIN) and output (VOUT) voltages determine the

switch duty cycle (D), which, in turn, is used to determine the

inductor ripple current.

D = VOUT − VIN

VOUT

Use the duty cycle and switching frequency (fSW) to determine

the on time.

t ON

=

D

f SW

The inductor ripple current (ΔIL) in a steady state is:

∆I L

=

VIN

× t ON

L

Solve for the inductance value (L):

L = VIN × t ON

∆I L

Make sure that the peak inductor current (that is, the maximum

input current plus half of the inductor ripple current) is less

than the rated saturation current of the inductor. In addition,

ensure that the maximum rated rms current of the inductor is

greater than the maximum dc input current to the regulator.

For duty cycles greater than 50% that occur with input voltages

greater than half the output voltage, slope compensation is required

to maintain stability of the current mode regulator. The inherent

open-loop stability causes subharmonic instability when the

duty ratio is greater than 50%. To avoid subharmonic instability,

the slope of the inductor current should be less than half of the

compensation slope.

Inductor manufacturers include: Coilcraft, Inc., Sumida

Corporation, and Toko.

Input and Output Capacitor Selection

The ADD5207 requires input and output bypass capacitors to

supply transient currents while maintaining a constant input

and output voltage. Use a low effective series resistance (ESR)

10 μF or greater capacitor for the input capacitor to prevent noise

at the ADD5207 input. Place the input between VIN and GND,

as close as possible to the ADD5207. Ceramic capacitors are

preferred because of their low ESR characteristics. Alternatively,

use a high value, medium ESR capacitor in parallel with a

0.1 μF low ESR capacitor as close as possible to the ADD5207.

The output capacitor maintains the output voltage and supplies

current to the load while the ADD5207 switch is on. The value

and characteristics of the output capacitor greatly affect the

output voltage ripple and stability of the regulator. Use a low

ESR output capacitor; ceramic dielectric capacitors are preferred.

For very low ESR capacitors, such as ceramic capacitors, the

ripple current due to the capacitance is calculated as follows.

Because the capacitor discharges during the on time (tON), the

charge removed from the capacitor (QC) is the load current

multiplied by the on time. Therefore, the output voltage ripple

(ΔVOUT) is

∆VOUT

= QC

C OUT

= I L × t ON

C OUT

where:

COUT is the output capacitance.

IL is the average inductor current.

Rev. A | Page 12 of 16

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