MAX8729EEI+T Maxim Integrated Products, MAX8729EEI+T Datasheet - Page 24

• Manufacturer Part Number: MAX8729EEI+T
• Description: Display Drivers Constant-Frequency H alf-Bridge CCFL Inve
• Manufacturer: Maxim Integrated Products

Specifications of MAX8729EEI+T

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Parallel capacitor C3 sets the maximum operating fre-
quency, which is also the parallel-resonant peak fre-
quency. Choose:
In the circuit of Figure 1, the maximum resonant fre-
quency is 70kHz, C1 and C2 are 2.2µF, and the sec-
ondary leakage inductance is 300mH. Therefore, use a
capacitor of 12pF or greater for C3.
The transformer core saturation should also be consid-
ered when selecting the operating frequency. The pri-
mary winding should have enough turns to prevent
transformer saturation under all operating conditions.
Use the following expression to calculate the minimum
number of turns N1 of the primary winding:
where D
0.8) of the high-side switch, V
DC input voltage, B
core, and A is the minimal cross-section area of the core.
The COMP capacitor sets the speed of the current-reg-
ulation loop that is used during startup, maintaining
lamp-current regulation, and during transients caused
by changing the input voltage. To maintain stable oper-
ation, the COMP capacitor (C
least 3.3nF.
As discussed in the Digital PWM Dimming Control sec-
tion, the COMP capacitor also limits the dynamics of
the lamp-current envelope in digital PWM operation. At
the end of the digital PWM ON cycle, the MAX8729
turns on a 100µA internal current source to linearly dis-
charge the COMP capacitor. Use the following equa-
tion to set the fall time:
where t
and 1.5V is the dynamic range of the COMP voltage. At
the beginning of the digital PWM ON cycle, the COMP
capacitor is charged by transconductance error ampli-
fier, so the charge current is not constant. Because the
Constant-Frequency, Half-Bridge CCFL
Inverter Controller
24
______________________________________________________________________________________
FALL
MAX
C
3
is the fall time of the lamp-current envelope
is the maximum duty cycle (approximately
≤
C
N
4
COMP
1 >
π
2
S
x f
D
COMP Capacitor Selection
is the saturation flux density of the
MAX
B x A x f
=
2
S
MAX
100
C
1
x V
µ
+
x L x C
A x t
1 5 .
IN MAX
C
IN(MAX)
COMP
(
MIN
2
V
FALL
S
)
) needs to be at
−
is the maximum
N
2
average charge current is around 30µA, the rise time is
about three times longer than the fall time.
The TFLT capacitor determines the delay time for both
the open-lamp fault and secondary short-circuit fault.
The MAX8729 charges the TFLT capacitor with a 1µA
current source during an open-lamp fault and charges
the TFLT capacitor with a 126µA current source during
a secondary short-circuit fault. Therefore, the sec-
ondary short-circuit fault-delay time is approximately
100 times shorter than that of the pen-lamp fault. The
MAX8729 sets the fault latch when the TFLT voltage
reaches 4V. Use the following equations to calculate
the open-lamp fault delay (T
ondary short-circuit fault delay (T
The high-side gate driver is powered using a bootstrap
circuit. The MAX8729 integrates the bootstrap diode so
only one 0.1µF bootstrap capacitor is needed. Connect
the capacitor between LX and BST to complete the
bootstrap circuit.
Careful PC board layout is important to achieve stable
operation. The high-voltage section and the switching
section of the circuit require particular attention. The
high-voltage sections of the layout need to be well sep-
arated from the control circuit. Follow these guidelines
for good PC board layout:
1) Keep the high-current paths short and wide, espe-
2) Use a star-ground configuration for power and ana-
cially at the ground terminals. This is essential for
stable, jitter-free operation and high efficiency.
log grounds. The power and analog grounds
should be completely isolated—meeting only at the
center of the star. The center should be placed at
the analog ground pin (GND). Using separate cop-
per islands for these grounds can simplify this task.
Quiet analog ground is used for V
LF, and TFLT.
T
T
OPEN LAMP
SEC SHORT
Setting the Fault-Delay Time
_
_
=
=
Bootstrap Capacitor
C
C
TFLT
TFLT
OPEN_LAMP
126
Layout Guidelines
SEC_SHORT
1
µ
A
µ
x V
x V
A
4
4
CC
, COMP, HF,
) and sec-
):