MAX1996AETI+ Maxim Integrated Products, MAX1996AETI+ Datasheet - Page 19

• Manufacturer Part Number: MAX1996AETI+
• Description: Display Drivers CCFL Backlight Controller
• Manufacturer: Maxim Integrated Products

Specifications of MAX1996AETI+

• Lead Free Status / Rohs Status: Details

IFB is the half-wave rectified representation of the cur-
rent through the lamp. The inverter regulates the aver-
age voltage at IFB, which is controlled by either the
analog interface or the SMBus interface. To set the
maximum lamp RMS current, determine R1 as follows:
R1 = 0.444V/I
is the maximum RMS lamp current. MINDAC and the
wave shape influence the actual maximum RMS lamp
current. If necessary, use an RMS current meter to
make final adjustments to R1.
The MAX1996A limits the transformer secondary voltage
during open-lamp fault through the capacitive divider
C3/C4. The voltage of V
age. To set the maximum RMS secondary transformer
voltage, choose C3 around 10pF to 22pF, and select C4
such that C4 = V
prises the maximum RMS secondary transformer voltage
(above the strike voltage). R2 sets the V
to zero volts. Choose R2 =10/(C4
F
CCI sets the speed of the current loop that is used dur-
ing startup, maintaining lamp-current regulation, and
during transients, caused by changing the lamp-cur-
rent settling. The typical CCI capacitor value is 0.1µF.
Larger values limit lamp-current overshoot, but
increase setting time. Smaller values speed up its
response time, but extremely small values can lead to
instability.
CCV sets the speed of the voltage loop that affects start-
up, DPWM transients, and operation in an open-tube
fault condition. If DPWM is not used, the voltage control
loop should only be active during startup or an open-
lamp fault. The CCV capacitors typical value is 0.01µF.
Use the smallest value of CCV capacitor necessary to
set an acceptable fault-transient response and not cause
excessive ringing at the beginning of a DPWM pulse.
Larger CCV capacitor values reduce transient overshoot,
but can degrade regulation at low DPWM duty cycles by
increasing the delay to strike voltage.
The MAX1996A works well with air-gap transformers
with turns ratio N in the order of N
for most applications. The transformer secondary reso-
nant frequency must be controlled. A low-profile CCFL
transformer typically operates between 50kHz (F
and 200kHz (F
capacitor C2, parallel capacitor C3, and the CCFL
lamp form a resonant tank. The resonant frequency is
SW
is the nominal resonant operating frequency.
CCFL
T(MAX)
max
______________________________________________________________________________________
,
). Transformer T1, DC blocking
RMS
Voltage-Sense Capacitors
/1.11V
FB
,
MAX
Resonant Components
is proportional to CCFL volt-
✕
Loop Compensation
, where I
C3, where V
✕
Range, CCFL Backlight Controller
P
:N
6.28
High-Efficiency, Wide Brightness
S
FB
CCFL
= 1:90 to 1:100
✕
DC bias point
F
T(MAX)
,
SW
RMS
), where
,
com-
MAX
min
)
determined by the transformer secondary leakage
inductance L, C2, and C3. The tank is a bandpass filter
whose lower frequency is bounded by L, N, and C2. N
is the transformer’s turns ratio. Choose C2 ≤ N
F
C3. Choose C3 ≥ 1/(40
The high-side MOSFET drivers (GH1 and GH2) are
powered by the external bootstrap circuit formed by
D2, C5, and C6. Connect BST1/BST2 through a dual
signal-level Schottky diode D2 to V
LX1/LX2 with 0.1µF ceramic capacitors. Use a dual-
series signal-level diode (D1) to generate the half-wave
rectified current-sense voltage across R1. The current
through these diodes is the lamp current.
The MAX1996A can be used to drive two CCFL tubes
as shown in Figure 12. See Table 5 for component
selection. The circuit consists of two identical trans-
formers with primary windings connected in parallel
and secondary windings in series. The two transform-
ers can also be replaced with a single transformer,
which has one primary winding and two secondary
windings. The advantage of the series secondary wind-
ings is that the same current flows through both lamps,
resulting in approximately the same brightness.
In normal operation, C12 is charged to approximately
6V biasing N1 on, which permits current to flow in the
loop as follows: in the first half cycle, current flows
through the secondary winding of T1, CCFL1, diode
D1, MOSFET N1, sense resistor R1, zener diode D4
(forward bias), CCFL2, and finally returning to T2. In the
second half cycle, the lamp current flows through T2,
CCFL2, D4 (breakdown), D3 (forward bias), CCFL1,
and back to T1.
The roundabout path of current flow is necessary in
order to detect an open-lamp condition when either
CCFL is removed. If CCFL1 is open, the lamp current
cannot flow through sense resistor R1. When IFB drops
below 150mV, the controller detects the condition and
shuts down after a 1s delay. During the delay, current
can flow from T2 through CCFL2, D4 (breakdown), and
R6 back to T2. If CCFL2 is removed, the voltage across
D4 drops to zero and C11 is discharged through R5.
N1 is biased off, which forces the voltage at IFB to drop
to zero once again. During the 1s turn-off delay, current
flows from T1 to CCFL1 through D3 (breakdown) and
R6 back to T1. D3 clamps the drain of N1 enabling the
use of a MOSFET with modest breakdown characteristics.
2 MIN
✕
L). The upper frequency is bounded by L and
✕
F
2 MIN
Dual-Lamp Regulator
Other Components
✕
L).
DD
, and connect it to
2
(10
19
✕