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參數(shù)資料
| 型號(hào)： | MAX1610 |
| 廠商： | Maxim Integrated Products, Inc. |
| 英文描述： | Digitally Controlled CCFL Backlight Power Supplies |
| 中文描述： | 數(shù)控CCFL背光燈電源 |
| 文件頁(yè)數(shù)： | 12/20頁(yè) |
| 文件大小： | 137K |
| 代理商： | MAX1610 |
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M
Digitally Controlled CCFL Bac klight
Power S upplies
12
______________________________________________________________________________________
Figure 8. Transformer Primary/Secondary Voltage Relationship
T
V
T
V
2
π
ω
π
V
CT
2
2
π
ω
N
π
V
CT
2
-N
π
V
CT
2
NOTE: V
CT
= AVERAGE VOLTAGE FROM THE T1 CENTER TO THE EMITTERS OF Q1 AND Q2 (IGNORING Q1, Q2 V
CE, SAT
).
ω
= 2
π
f
ROY
.
Open-Tube Protection (OTP)
Any real transformer used in a Royer oscillator will have a
maximum-allowed secondary voltage. If the maximum-
allowed secondary voltage is exceeded, the winding
insulation can break down, leading to permanent trans-
former damage. The maximum-allowed secondary volt-
age can be exceeded either when the CCFL drive circuit
is turned on without the CCFL being in place, or when
the CCFL becomes disconnected during normal opera-
tion due to a mechanical failure. To protect against these
fault conditions, use the OTP pin to sense the voltage on
the transformer center tap (pin 2 of Figure 4). Whenever
the voltage on OTP exceeds the REF reference voltage,
the BATT-to-LX power switch is forced off.
For example, in Figure 4, the CTX110605 transformer
has a maximum-allowed continuous secondary voltage
of 1340V
RMS.
D1 and C5 detect the peak voltage on
the center tap of T1. R3 and R4 determine the limit on
the center tap peak voltage. The relationship between
the voltage on the center tap of T1 and the secondary
voltage is diagrammed in Figure 8. Neglecting the
Q1/Q2 saturation voltage and the voltage on the R1
current-sense resistor yields Equation 1:
where V
SEC
is the maximum root-mean-square voltage
allowed on the secondary, N is the secondary-to-prima-
ry turns ratio, and V
CTPK
is the peak voltage on the
transformer center tap.
Bloc k Diagram of the Analog S ec tion
Figure 9 shows a functional diagram of the analog cir-
cuitry in the MAX1610/MAX1611. The chips have identi-
cal analog circuitry, and differ only in their digital
interface.
Loop-Compensation Capacitor (CC)
The BATT-to-LX switch turns on at fixed frequency, and
turns off when the current-sense voltage on the CS pin
exceeds CC - REF. As the CC pin voltage rises, the CS
current limit rises as well. A transconductance amplifier
compares the voltage on CSAV to the desired regulation
voltage and outputs a current proportional to this error
to the CC pin. A capacitor from CC to GND sets the
bandwidth of this regulation loop, as shown in Equation 2:
where BW is the bandwidth of the CSAV regulation loop
in kHz, and C3 is the capacitance from CC to GND
in nF.
Soft Start (SS)
Soft start prevents the triggering of OTP upon power-
up. Placing a capacitor from SS to GND soft starts the
Royer oscillator by slowly raising the CS current-limit
voltage. Internal circuitry pulls SS to GND during
power-on reset, or whenever the lamp is turned off (DAC
= 00000, shutdown mode, ON-1 = 0, or ON-0 = 0)
(Figures 10 and 11). When SS is not pulled to GND, an
internal 4μA current sources into the capacitor at the
SS pin. This pin is internally diode clamped to REF so
that it rises to a maximum voltage of about 2.7V.
Regardless of the voltage on CC, the CS current-sense
voltage is never allowed to exceed the voltage on SS
divided by 5.
Frequency Selection and Synchronization
The SYNC pin performs two functions: it sets the BATT-
to-LX switching frequency, and it allows the BATT-to-LX
switching frequency to be synchronized to an external
oscillator. SYNC tied to GND or VL sets a 145kHz
switching frequency; SYNC tied to REF sets a 290kHz
BW =
85
2 C3
V
= V
2
2N
CTPK
SEC
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