MAX753/MAX754
CCFL Backlight and
LCD Contrast Controllers
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Positive LCD Bias: MAX754
The voltage-regulation loop is comprised of resistors R3
and R4, the pulse-skip comparator, the internal DAC,
the on-time and off-time logic, and the external power
components. The comparator compares a fraction of
the output voltage to the voltage generated by an on-
chip 6-bit DAC. The part regulates by keeping the volt-
age at LFB equal to the DAC’s output voltage. Thus,
you can set the output to different voltages by varying
the DAC’s output.
Varying the DAC output voltage (digital control) adjusts
the external voltage from 50% to 100% of full scale. On
power-up or after a reset, the counter sets the DAC out-
put to mid scale. Each rising edge of LADJ (with LON
high) decrements the DAC output. When decremented
beyond zero scale, the counter rolls over and sets the
DAC to the maximum value. In this way, a single pulse
applied to LADJ decreases the DAC set point by one
step, and 63 pulses increase the set point by one step.
The MAX754’s DAC transfer function is shown in Figure 7.
The following equation relates the switching regulator’s
regulated output voltage to the DAC’s voltage:
Table 5 is the logic table for the LADJ and LON inputs,
which control the internal DAC and counter. As long as the
timing specifications for LADJ and LON are observed, any
sequence of operations can be implemented.
Negative LCD Bias: MAX753
The LCD bias generator of the MAX753 (Figure 8) gen-
erates its negative output by combining the switching
regulator of the MAX754 with a simple diode-capacitor
voltage inverter. To best understand the circuit, look at
the part in a steady-state condition. Assume, for
instance, that the output is being regulated to -30V, and
that the battery voltage is +10V. When Q3 turns on, two
things occur: current ramps up in the inductor, just like
with the boost converter; and the charge on C15 (trans-
ferred from the inductor on the previous cycle) is trans-
ferred to C6, boosting the negative output. At the end of
the cycle, the voltage on C15 is 30V + Vd, where Vd is
the forward voltage drop of Schottky diode D3, and 30V
is the magnitude of the output.
When the MOSFET turns off, the inductor’s energy is
transferred to capacitor C15, charging the capacitor to
a positive voltage (V
HIGH
) that is higher than
|V
OUT
|
. In
this instance, diode D8 allows current to flow from the
right-hand side of the flying capacitor (C15) to ground.
When the MOSFET turns on, the left-hand side of
capacitor C15 is clamped to ground, forcing the right-
hand side to -V
HIGH
. This voltage is more negative than
the output, forcing D3 to conduct, and transferring
charge from the flying capacitor C15 to the output
capacitor C6. This charge transfer happens quickly,
resulting in a voltage spike at the output due to the
product of the output capacitor’s equivalent series
resistance (ESR) and the current that flows from C15 to
C6. To limit this drop, resistor R19 has been placed in
series with D3. R19 limits the rate of current flow. At the
end of this cycle, the flying capacitor has been dis-
charged to 30V + Vd.
If BATT(MAX) (i.e., either the fully charged battery volt-
age, or the wall-cube voltage) is greater than
|V
OUT
(MIN)|, tie the cathode of D8 to BATT instead of
GND, as shown by the dashed lines in Figure 8.
Efficiency is lower with this method, so tie the cathode
of D8 to GND whenever possible.
The MAX753’s regulation loop is similar to that of the
MAX754. The MAX753, however, uses different power
components, and its feedback resistors are returned to
the reference (1.25V) rather than ground.
The MAX753’s PFM comparator compares a fraction of
the output voltage to the voltage generated by the on-
chip 6-bit DAC. The part regulates by keeping the volt-
age at LFB equal to the DAC’s output voltage. Thus,
you can set the LCD bias voltage to different voltages
by varying the DAC’s output.
