MAX8751ETJ+ Maxim Integrated Products, MAX8751ETJ+ Datasheet - Page 14

• Manufacturer Part Number: MAX8751ETJ+
• Description: IC CNTRLR CCFL INV 32-TQFN
• Manufacturer: Maxim Integrated Products
• Type: CCFL Controllerr

Specifications of MAX8751ETJ+

• Frequency: 30 ~ 80 kHz
• Current - Supply: 3.2mA
• Voltage - Supply: 6 V ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 32-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Lead Free Status / Rohs Status: Details

Fixed-Frequency, Full-Bridge CCFL
Inverter Controller
Figure 1 shows the Stand-Alone Typical Operating
Circuit and Figure 2 shows the Functional Diagram of
the MAX8751. The circuit in Figure 1 consists of a full-
bridge inverter, which converts unregulated DC input
voltage into a nearly sinusoidal high-frequency, AC out-
put for powering CCFLs. The MAX8751 is biased from
an internal 5.35V linear regulator with UVLO compara-
tor that ensures stable operation and clean startup
characteristics. The MAX8751 includes several layers
of fault-protection circuitry, consisting of comparators
for detecting primary-side current limit, secondary-side
overvoltage, secondary short circuit, and open-lamp
faults. A logic block arbitrates the comparator outputs
by making sure that a given fault persists for a mini-
mum duration before registering the fault condition. A
separate block provides dimming control based upon
analog or DPWM inputs. Finally, a dedicated logic cir-
cuit provides synchronization and phase-control func-
tions for daisy-chaining up to five MAX8751s without
phase overlap.
The MAX8751 operates in resonant mode during strik-
ing and switches over to constant-frequency operation
after the IFB voltage rises above the open-lamp thresh-
old. Reliable striking of all lamps is ensured by using
individual transformer secondary winding for each
lamp, or by using ballast capacitors if multiple lamps
are driven by single transformer secondary. The con-
stant-frequency architecture supported by the
MAX8751 can be synchronized and phase shifted for
daisy-chained applications. Multiple lamps can also be
driven in parallel within a single stage. The MAX8751
has sufficient gate drive strength to drive the large-
power MOSFETs needed when one power stage drives
four or more CCFL lamps in parallel.
The MAX8751 provides accurate lamp-current regula-
tion. A primary-side current sense provides cycle-by-
cycle current limit and zero-crossing detection, while the
lamp current is sensed with a separate loop that pro-
vides fine adjustment of the lamp current with an external
resistor. The MAX8751 controls lamp brightness by turn-
ing the CCFL on and off using a DPWM method, while
maintaining approximately constant lamp current. The
brightness set point can be adjusted with an analog volt-
age on the CNTL pin, or with an external PWM signal.
The MAX8751 has a single compensation input
(COMP), which also establishes the soft-start and soft-
stop timing characteristics. Control logic changes the
available drive current at COMP based on the operat-
ing mode to adjust the inverter’s dynamic behavior.
14
______________________________________________________________________________________
Detailed Description
The MAX8751 operates in constant-frequency mode in
normal operation. There are two ways to set the switch-
ing frequency:
1) The switching frequency can be set with an external
The adjustable range of the switching frequency is
between 20kHz and 100kHz (R
and 54kΩ).
2) The switching frequency can be synchronized by
The frequency range of the external signal should be
between 120kHz and 600kHz, resulting in a switching
frequency range between 20kHz and 100kHz.
Figure 3 is the timing diagram of constant-frequency
operation, showing the primary current, internal oscilla-
tor, and gate signals. At the beginning of the positive
half cycle, switches NH1 and NL2 are ON (see Figure
1), and the primary current ramps up. The controller
turns off NH1 when the primary current reaches it peak,
which is set by the COMP voltage. The primary current
continues to flow through the freewheeling body diode
of NL1. Next, the low-side switch NL1 is turned on
under zero-voltage switching (ZVS) conditions. Now the
primary current starts ramping down. The falling edge
of the internal oscillator turns off NL2 and turns on NH2,
starting the negative half cycle. The fixed-frequency
operation continues with the inverter controlling the full-
bridge MOSFETs to produce near the sinusoidal lamp-
current waveform.
The MAX8751 operates in resonant mode during start-
up. In the resonant mode, the switching frequency is
synchronized with the natural resonant frequency of the
resonant tank circuit. The synchronization and phase-
shift functions are disabled during startup. Figure 4 is
the timing diagram of the resonant operation showing
the primary current and gate signals. In the resonant
resistor connected between HF and GND. The switch-
ing frequency is given by the following equation:
an external high-frequency signal. Connect HF to
GND through a 100kΩ resistor and connect
HSYNC to the external high-frequency signal. The
resulting switching frequency (f
quency of the external signal (f
Constant-Frequency Operation
f
SW
f
SW
=
54
=
kHz x
f
SYNC
6
100
R
HF
Resonant Startup
HF
SYNC
k
Ω
is between
SW
) is 1/6 the fre-
):
270kΩ