Rainbow Electronics MAX754 User Manual

Page 9

MAX753/MAX754

CCFL Backlight and

LCD Contrast Controllers

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Table 1. CCFL Circuit Component Descriptions (continued)

ITEM

DESCRIPTION

C10

The ballast capacitor linearizes the CCFL impedance and guarantees no DC current through the lamp. 15pF
will work with just about any lamp. Depending on the lamp, you can try higher values, but this may cause the
regulation loop to become unstable. Larger values of C10 allow the circuit to operate with lower input volt-
ages. Don’t forget that C10 must be a high-voltage capacitor and cannot be polarized. A lamp with a
1500V

RMS

maximum strike voltage will require C10 to withstand 1500 x

√

2 = 2121V.

T1 must have high primary inductance (greater than 30µH), otherwise an inflated value of C9 will be required
in order to keep the Royer frequency below 60kHz (the maximum allowed by most lamps). A higher T1 sec-
ondary-to-primary turns ratio allows lower-voltage operation, but increases the size of the transformer.

You must select a value for C9 high enough to keep the lamp current reasonably sinusoidal and yet low
enough that T1’s core does not saturate. For the Sumida EPS207 with a 171:1 turns ratio, choose a 0.22µF

value for C9. The characteristic impedance of the resonant tank equals

, where L

MAG

is the mag-

netizing inductance of T1. The characteristic impedance is defined as the ratio of the voltage across the par-
allel LC circuit divided by the current flowing between the inductor and capacitor. This circulating current is
not delivered to the load. If C9 has too large a value, it will cause excessive circulating currents, which will in
turn saturate the core of T1. It’s easy to tell when you have excess circulating current in the resonant tank,
because when you touch T1 you burn your finger. However, reducing the value of C9 decreases tank Q,
which increases the harmonic content of the lamp-current waveform. If the lamp-current waveform does not
look sinusoidal, then the circuit may not regulate to the right root mean square current.

MAG

R10

R10 sets the base current for Q4 and Q5. If you choose too large a value for R10, Q4 and Q5 will overheat.
Too small a value will waste base current and slightly degrade efficiency. The optimal value will depend on
how much power you are trying to deliver to the lamp. 510

Ω

is a good “always works but may not be the most

efficient” value for use with the FMMT619 transistors from ZETEX.

R5, R6

This resistive divider senses the voltage at the center tap of T1. When the CC pin on the MAX758A rises
above 1.25V, the internal switch turns off, interrupting power to the Royer oscillator and limiting the open-lamp
transformer center-tap voltage.

D6B, C7, R7

D6B, C7, and R7 form a soft-start clamp, which limits the rate-of-rise of the peak current in the MAX758A.
Make sure R7 is at least 100k

Ω

so it does not excessively load the CC pin.

D6A, R17

D6A and R17 are also part of the soft-start clamp. The voltage on the SS pin controls the peak current in the
MAX758A’s switch. Make sure R17 is at least 100k

Ω

so it does not excessively load the CC pin.

Inductor for the Switching-Current Source. Use a 47µH to 150µH inductor with a 1A to 1.5A saturation current.

Schottky Catch Diode. Use a 1A to 1.5A Schottky diode with low forward-voltage power.

Supply Bypass Capacitor. Use low-ESR capacitor.