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参数资料
| 型号： | MAX8785ETI+T |
| 厂商： | Maxim Integrated |
| 文件页数： | 14/19页 |
| 文件大小： | 0K |
| 描述： | IC CCFL CTRL 28-TQFN |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 2,500 |
| 类型： | CCFL 控制器 |
| 电流 - 电源： | 1.5mA |
| 电源电压： | 4.5 V ~ 28 V |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 28-WFQFN 裸露焊盘 |
| 供应商设备封装： | 28-TQFN-EP（5x5） |
| 包装： | 带卷 (TR) |
Full-Bridge Controller for
Piezoelectric Transformers
The frequency of the internal DPWM oscillator is
adjustable through a resistor connected between LF
and GND. The DPWM frequency is given by:
f DPWM ≈ 208 Hz × 150 k Ω / R LF
The adjustable range of the DPWM frequency is 100Hz
< f DPWM < 300Hz, with a corresponding programming
resistance of 103k Ω < R LF < 315k Ω .
External Digital (DPWM) Control
Connect SEL to V CC and an external digital signal at
LSYNC to enable digital control mode. DPWM duty ratio
and frequency are the same as LSYNC duty ratio and
frequency. The frequency range of the digital signal is
120Hz to 280Hz. The range of duty ratio for LSYNC is
10% to 100%, and for correct lamp operation, the duty
cycle should always be above 10%. When the duty
ratio of LSYNC is 100%, the CCFL is at full brightness.
A phase-lock loop (PLL) is used to synchronize the
internal DPWM signal with the externally applied digital
signal at LSYNC. PLL is a feedback system that oper-
ates on the excess phase of a periodic signal. Connect
a capacitor from PCOMP to GND to stabilize the PLL.
To ensure fast response of the PLL, connect a 10nF
capacitor from PCOMP to GND.
Lamp-Current Regulation
The MAX8785A uses a lamp-current control loop to reg-
ulate the CCFL current. The control loop is a transcon-
ductance amplifier as shown in Figure 2. The AC lamp
current is sensed with a sense resistor connected in
series with the low-voltage terminal of the lamp. The IFB
input is internally full-wave rectified. The transconduc-
tance error amplifier compares the average value of the
rectified IFB voltage with 800mV (typ) internal reference.
The output of the transconductance error amplifier
V COMP controls a VCO, which sets the switching fre-
quency of the inverter.
Lamp Startup
A CCFL is a gas-discharged lamp that is normally dri-
ven in the avalanche mode. To start ionization in a non-
ionized lamp, the applied voltage (striking voltage) must
be increased to the level required to start the flow of
current. For example, the normal running voltage of a
typical CCFL is approximately 650V RMS , but the striking
voltage can be as high as 1800V RMS .
The MAX8785A’s control architecture ensures striking of
the lamp. As the COMP voltage rises, the VCO sweeps
the switching frequency from the switching frequency
(set by the R HF resistor) to the point where the gain of
resonant tank is sufficient to strike the lamp. At startup,
the MAX8785A overrides the external DPWM setting
and forces 100% brightness setting until the lamp
strikes and the lamp current reaches regulation. After
the current reaches regulation, the MAX8785A switches
to normal DPWM operation using the brightness control
mode defined by the SEL pin.
Secondary Short-Circuit Protection
The MAX8785A provides protection against short cir-
cuit at the high-voltage terminal of the PZT or excessive
leakage from a high-voltage terminal to ground. The
MAX8785A senses secondary voltage through the VFB
pin (see Figure 1). If the sensed voltage stays below
the 230mV threshold for more than the fault time set by
the TFLT cap, the MAX8785A disables the gate drivers
to avoid excessive output current. The fault time is
determined by charging the TFLT capacitor with a
2.8mA current to 3V. When the TFLT fault latch is set,
the MAX8785A stops switching and FLT is pulled low.
UVLO
The MAX8785A includes a V CC undervoltage-lockout
(UVLO) feature. If V CC is below 4.12V (typ), the high-
side and low-side switch gate drivers are disabled and
the fault latch is set.
Low-Power Shutdown
When V SHDN < 0.8V, all functions of the IC are turned off
except the V CC . In shutdown, the linear-regulator output
voltage is 4.6V (typ) and the supply current is 10μA
(typ). While in shutdown, the arc protection, lamp-out
detection, and short-circuit detection latches are reset.
Lamp-Out Detection and
Overvoltage Protection
The IFB pin monitors the lamp current to detect faulty or
open CCFL lamps. If the peak IFB voltage is less than
800mV, a fault is detected and the MAX8785A charges
the TFLT capacitor with a 0.75μA current source. If the
voltage on TFLT exceeds 3V, the fault latch is set.
During the lamp-out detection period, the MAX8785A
decreases the switching frequency in an effort to strike
the lamp. This can result in very high secondary voltage.
To address this problem, the MAX8785A includes an
overvoltage-protection circuit. The lamp voltage is
sensed at the VFB pin, and once the secondary voltage
exceeds the overvoltage threshold of 2.25V (typ), an
internal 1.0mA current source discharges the COMP
node. When COMP discharges, the inverter’s switching
frequency increases, thereby reducing the gain of the
resonant tank and limiting the secondary voltage.
14
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