Wide Brightness Range

CCFL Backlight Controllers

16

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optimized using R14 and R15 as shown in Figure 7.

The resistor-divider can be used to force commutation

as close to the zero-crossing point as possible.

POR and UVLO

The MAX1739/MAX1839 include power-on reset (POR)

and undervoltage lockout (UVLO) features. The POR

resets all internal registers, such as DAC output, fault

conditions, and all SMBus registers. POR occurs when

VL is below 1.5V. The SMBus input logic thresholds are

designed to meet electrical characteristic limits for VL

as low as 3.5V, but the interface will continue to func-

tion down to the POR threshold.

The UVLO threshold occurs when VL is below 4.2V

(typ) and disables the buck-switch driver.

Low-Power Shutdown

When the MAX1739/MAX1839 are placed in shutdown,

all IC functions are turned off except the 5V linear regu-

lator that powers all internal registers and the SMBus

interface (MAX1739). The SMBus interface is accessi-

ble in shutdown. In shutdown, the linear regulator out-

put voltage drops to about 4.5V and the supply current

is 6µA (typ), which is the required power to maintain all

internal register states. While in shutdown, lamp-out

detection and buck-switch short-circuit detection latch-

es are reset. The device can be placed into shutdown

by either writing to the MODE register (MAX1739

SMBus mode only) or with SH/SUS.

Lamp-Out Detection

For safety, during a lamp-out condition, the MAX1739/

MAX1839 limit the maximum average primary-side

transformer voltage (see Sense Resistors) and shut

down the lamp after 2s.

(typ) within 2 seconds. This circuitry ignores most puls-

es under 200ns. However, in some cases, a small

capacitor is needed at CSAV to prevent noise from trip-

ping the circuitry. This is especially true in noisy envi-

ronments and in designs with marginal layout.

Ideally, the voltage at CSAV is a half-wave rectified sine

wave. In this case, the CSAV lamp-out threshold is as

follows:

the maximum lamp current (see Sense Resistors). Note:

The formulas assume a worst-case CSAV lamp-out

threshold of 100mV and a maximum CSAV average

voltage of 200mV.

Use MINDAC or limit the brightness code to prevent

setting the lamp current below the CSAV lamp-out

threshold.

STATUS1 bit sets when the lamp-out detection circuit

shuts down the device.

Buck-Switch Short Fault Detection

and Protection

When the buck switch (N1) fails short, there is no volt-

age limiting on the transformer and the input forces

excessive voltage on the transformer secondary. This

Figure 6. DPWM Synchronization

MODE

GND

DPWM

SYNCHRONIZATION

SIGNAL

10k

500pF

ADC-

ADC+

SMBus

VL

REF

MAX1739

MAX1839

Figure 7. Adjusting the ZVC Detection

SYNC

R14

R15

R4

R5

REF

CTFB

LX

L1

T1

GND

MAX1739

MAX1839