參數(shù)資料
型號(hào)： ISL97522
廠商： Intersil Corporation
英文描述： 4-Channel TFT-LCD Supply
中文描述： 4通道的TFT - LCD電源
文件頁數(shù)： 15/20頁
文件大?。?/td> 418K
代理商： ISL97522
15
FN7445.0
December 13, 2006
Discontinuous/Continuous Boost Operation and
its Effect on the Charge Pumps
The ISL97522 V
ON
and V
OFF
architecture uses LX
switching edges to drive diode charge pumps from which
LDO regulators generate the V
ON
and V
OFF
supplies. It can
be appreciated that should a regular supply of LX switching
edges be interrupted, for example during discontinuous
operation at light A
VDD
boost load currents, then this may
affect the performance of V
ON
and V
OFF
regulation -
depending on their exact loading conditions at the time.
To optimize V
ON
/V
OFF
regulation, the boundary of
discontinuous/continuous operation of the boost converter
can be adjusted, by suitable choice of inductor given V
IN
,
V
OUT
, switching frequency and the A
VDD
current loading, to
be in continuous operation.
The following equation gives the boundary between
discontinuous and continuous boost operation. For continuous
operation (LX switching every clock cycle) we require that:
I(A
VDD
_load) > D*(1-D)*V
IN
/(2*L*F
OSC
)
where the duty cycle, D = (A
VDD
- V
IN
)/A
VDD
For example, with V
IN
= 5V, F
OSC
= 1.0MHz and A
VDD
=
12V we find continuous operation of the boost converter can
be guaranteed for:
L = 10μH and I(A
VDD
) > 61mA
L = 6.8μH and I(A
VDD
) > 89mA
L = 3.3μH and I(A
VDD
) > 184mA
Buck Converter
The buck converter is the step down converter, which
supplies the current to the logic circuit of the LCD system. In
the continuous current mode, the relationship between input
voltage and output voltage is as following:
Where D is the duty cycle of the switching MOSFET.
Because D is always less than 1, the output voltage of buck
converter is lower than input voltage.
T
he
Feedback Resistors
The buck converter output voltage is determined by the
following equation:
Where R12 and R13 are the feedback resistors of buck
converter to set the output voltage Current drawn by the
resistor network should be limited to maintain the overall
converter efficiency. The maximum value of the resistor
network is limited by the feedback input bias current and the
potential for noise being coupled into the feedback pin. A
resistor network in the order of 300
Ω
is recommended.
Buck Converter Input Capacitor
The capacitor should support the maximum AC RMS current
which happens when D = 0.5 and maximum output current.
Where I
O
is the output current of the buck converter.
V
IN
OR A
VDD
CHARGE PUMP
OUTPUT
3k
Ω
Q3
FBP
ISL97522
DRVP
NPN
CASCODE
TRANSISTOR
V
ON
FIGURE 19. CASCODE NPN TRANSISTOR CONFIGURATION
FOR HIGH CHARGE PUMP OUTPUT VOLTAGE
(>36V)
V
ON
(>36V)
0.1μF
0.1μF
0.1μF
0.1μF
3k
Ω
.47μF
0.22μF
0.1μF
A
VDD
LX
Q3
FBP
ISL97522
DRVP
FIGURE 20. THE LINEAR REGULATOR CONTROLS ONE
STAGE OF CHARGE PUMP
V
IN
-------------------
D
=
(EQ. 16
V
LOGIC
R
--------------------------
R
+
13
V
FBL
×
=
(EQ. 17)
I
acrms
C
IN
(
)
D
1
D
–
(
)
I
O
=
(EQ. 18)
ISL97522
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