MAX1839EEP datasheet(11/26 Pages) MAXIM

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Wide Brightness Range

CCFL Backlight Controllers

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Voltage and Current Control Loops

The MAX1739/MAX1839 use two control loops. The cur-

rent control loop regulates the average lamp current. The

voltage control loop limits the maximum average primary-

side transformer voltage. The voltage control loop is

active during the beginning of DPWM on-cycles and in

some fault conditions. Limiting the transformer primary

voltage allows for a lower transformer secondary voltage

rating that can increase reliability and decrease cost of

the transformer. The voltage control loop acts to limit the

transformer voltage any time the current control loop

attempts to steer the transformer voltage above its limit as

set by VCTFB (see Sense Resistors).

The voltage control loop uses a transconductance

amplifier to create an error current based on the volt-

age between CTFB and the internal reference level

(600mV typ) (Figure 2). The error current is then used

to charge and discharge CCCV to create an error volt-

age VCCV. The current control loop produces a similar

signal based on the voltage between CSAV and its

internal reference level (see the Dimming Range sec-

tion). This error voltage is called VCCI. The lower of

VCCV and VCCI is used with the buck regulator’s PWM

ramp generator to set the buck regulator’s duty cycle.

During DPWM, the two control loops work together to

limit the transformer voltage and to allow wide dimming

range with good line rejection. During the DPWM off-

cycle, VCCV is set to 1.2V and CCI is set to high imped-

ance. VCCV is set to 1.2V to create soft-start at the

beginning of each DPWM on-cycle in order to avoid

overshoot on the transformer primary. VCCI is set to

high impedance to keep VCCI from changing during the

off-cycles. This allows the current control loop to regu-

late the average lamp current only during DPWM on-

cycles and not the overall average lamp current.

Upon power-up, VCCI slowly rises, increasing the duty

cycle, which provides soft-start. During this time, VCCV,

which is the faster control loop, is limited to 150mV

above VCCI by the CCV-CLAMP. Once the secondary

voltage reaches the strike voltage, the lamp current

begins to increase. When the lamp current reaches the

regulation point, VCCI reaches steady state. With MIN-

DAC = VL (DPWM disabled), the current control loop

remains in control and regulates the lamp current.

With MINDAC between REF and GND, DPWM is

enabled and the MAX1739/MAX1839 begin pulsing the

lamp current. During the on-cycle, VCCV is at 150mV

above VCCI. After the on-cycle, VCCV is forced down to

1.2V to provide soft-start at the beginning of the next

on-cycle. Also, VCCI retains its value until the beginning

of the next on-cycle. When VCCV increases, it causes

the buck regulator duty cycle to increase and provides

soft-start. When VCCV crosses over VCCI, the current

control loop regains control and regulates the lamp cur-

rent. VCCV is limited to 150mV above VCCI for the

remainder of the on-cycle.

In a lamp-out condition, VCCI increases the primary

voltage in an attempt to maintain lamp current regula-

tion. As VCCI rises, VCCV rises with it until the primary

voltage reaches its set limit point. At this point, VCCV

stops rising and limits the primary voltage by limiting

the duty cycle. Because VCCV is limited to 150mV

above VCCI, the voltage control loop is quickly able to

limit the primary voltage. Without this clamping feature,

the transformer voltage would overshoot to dangerous

levels because VCCV would take more time to slew

down from its supply rail. Once the MAX1739/MAX1839

sense less than 1/6 the full-scale current through the

lamp for 2 seconds, it shuts down the Royer oscillator

(see Lamp-Out Detection).

See the Sense Resistors section for information about

setting the voltage and current control loop thresholds.

Feed-Forward Control

Both control loops are influenced by the input voltage

feed-forward (VBATT) control circuitry of the MAX1739/

MAX1839. Feed-forward control instantly adjusts the

buck regulator’s duty cycle when it detects a change in

input voltage. This provides immunity to changes in

input voltage at all brightness levels. This feature

makes compensation over wide input ranges easier,

makes startup transients less dependent on input volt-

age, and improves line regulation for short DPWM on-

times.

The MAX1739/MAX1839 feed-forward control is imple-

mented by varying the amplitude of the buck-switch’s

PWM ramp amplitude. This has the effect of varying the

duty cycle as a function of input voltage while maintain-

ing the same VCCI and VCCV. In other words, VBATT feed

forward has the effect of not requiring changes in error-

signal voltage (VCCI and VCCV) to respond to changes in

VBATT. Since the capacitors only need to change their

voltage minimally to respond to changes in VBATT, the

controller’s response is essentially instantaneous.

Transient Overvoltage Protection

from Dropout

The MAX1739/MAX1839 are designed to maintain tight

control of the transformer primary under all transient

conditions. This includes transients from dropout,

where VBATT is so low that the controller loses regula-

tion and reaches maximum duty cycle. Backlight

designs will want to choose circuit component values to

minimize the transformer turns ratio in order to minimize

primary-side currents and I2R losses. To achieve this,

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