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参数资料
| 型号： | MAX8785ETI+T |
| 厂商： | Maxim Integrated |
| 文件页数： | 16/19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
Setting the Lamp Current
The MAX8785A senses the lamp current flowing through
resistor R1 (Figure 1) connected between the low-voltage
terminal of the lamp and ground. The voltage across R1
is fed to IFB and is internally full-wave rectified. The
MAX8785A controls the desired lamp current by regulat-
ing the average of the rectified IFB voltage. To set the
RMS lamp current, select R1 as follows:
Assuming the normal lamp operating voltage is 800V
and the resistor voltage rating is 200V, then n = 5.6.
Choose six resistors for the VFB string.
Setting the Arc Protection Threshold
If during normal operation, the PZT loses contact with
the PC board, the MAX8785A stops switching. This fea-
ture is referred to as arc protection. During normal
operation when the PZT-to-PC board connection is bro-
R 1 =
π × 800 mV
2 2 × I LAMP ( RMS )
ken, a very high voltage develops between the termi-
nals, resulting in arcing. The arcing is detected using a
capacitive voltage-divider from the PZT high-voltage
side to the OLF pin. Figure 7 shows an equivalent high-
where I LAMP(RMS) is the desired RMS lamp current,
and 800mV is the typical value of the IFB regulation
point. To set the RMS lamp current to 6mA, the value of
R1 should be 148 Ω . The closest standard 1% resistors
are 147 Ω and 150 Ω . The precise shape of the lamp-
current waveform depends on lamp parasitics. The
resulting waveform is an imperfect sinusoid waveform,
which has an RMS value that is not easy to predict. A
voltage capacitor between the bottom layer of the PZT
and the metal layer of the PC board. The lower layer of
the PZT and metal layer of the PC board creates a high-
voltage capacitor. Terminals 1 and 2 are the primary
side of the PZT, terminal 3 is the secondary side, and
terminal 4 is the metal layer of the PC board, which is
the low-voltage side of the capacitor (C PZT ).
high-frequency true RMS current meter (such as
Yokogawa 2016) should be used to measure the RMS
1
1
current and make final adjustments to R1. Insert this
meter between the sense resistor and the lamp’s low-
T1
3
T1
3
voltage terminal to measure the actual RMS current.
PC BOARD TRACE
Setting the Secondary Voltage Limit
2
4
2
4
The MAX8785A limits the transformer secondary volt-
age during lamp-out conditions. The secondary voltage
is sensed through a resistive voltage-divider, as shown
in Figure 1. The voltage at VFB is proportional to the
CCFL voltage. The total resistance from the HV side to
ground should be greater than 1M Ω so that the resis-
tive voltage-divider does not affect normal lamp opera-
Figure 7. Arc Protection
C PZT and C1 in Figure 1 form a capacitive voltage-
divider to OLF pin. These capacitors set the maximum
secondary voltage for an ARC fault. C1 can be calculat-
ed from the following equation:
tion. Resistors R2 and R3 through R9 set the maximum
secondary voltage limit. The resistance of R2 can be
calculated as follows :
C 1 =
2 × V LAMP ( RMS )_ MAX
1 . 2 V
× C PZT
R 2 =
V FB _ OV × R VFB
V LAMP _ MAX
C PZT should be measured on the board. Refer to the
MAX8785A EV Kit data sheet for suggested layout.
COMP Capacitor Selection
where R VFB = R3 + R4 + R5 + R6 + R7 + R8 + R9 =
1.4M Ω and VFB_OV is the overvoltage threshold. To set
the maximum lamp voltage to 2000V with R VFB =
1.4M Ω , R2 must be equal to 1.54k Ω . The voltage across
each resistor during normal operation should not
exceed its voltage rating; hence, the number of resistors
in R VFB can be calculated from the following equation:
COMP is the output of the transconductance error
amplifier for the lamp-current control loop. Connect a
capacitor between COMP and GND to stabilize the cur-
rent-control loop. The value of COMP capacitance
determines the response time of the lamp-current con-
trol loop. The COMP capacitance also determines the
power-on startup timing. The recommended COMP
n =
LampOperatingVoltage × 1.4
V SEC _ MAX
capacitance is 47nF.
16
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