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(yè)數(shù)： 12/20頁(yè)
文件大?。?/td> 418K
代理商： ISL97522
12
FN7445.0
December 13, 2006
f
S
is the switching frequency; D is the duty cycle.
To overcome the variation in external LX driver R
DS(ON)
, an
input is provided (ILADJ) to accommodate 5 different bands
of R
DS(ON)
by using 5 different selection resistors. Internally,
the ILADJ resistor adjusts two things:
1.the current limit;
2.the current feedback being used.
This keeps the dc-dc loop stable and the current limit the
same over a wide range of external drive FETs.
Alternatively, the current limit can be changed for the same
FET by varying the resistor. This would affect the stability of
the system somewhat (because the current feedback
changes) but be selected appropriately to accommodated
the change. The integrator loop should keep the load
regulation within limits as long as it doesn't run out of
dynamic adjustment range when current feedback gets
larger than intended. This could be determined by
measuring how close to the upper clamp limit the voltage on
the Cint pin voltage gets under maximum load current.
Here are the resistor settings on ILADJ which select the five
R
DS(ON)
ranges:
1/ 0ohms (Cfb factor 1, "Cfb" are the relative current
feedback factors)
2/ 30K (Cfb factor 1/1.8)
3/ 83K (Cfb factor 1/3.3)
4/ 182K (Cfb factor 1/5.7)
5/ >370K (Cfb factor 1/10)
1/ sets maximum internal current feedback and minimum
ILimit, used for low Ron fets.
5/ sets minimum internal current feedback and maximum
ILimit, used for large Ron fets.
The Current limit factors should be the inverse of the Cfb
values.
Input Capacitor
The input capacitor is used to supply the current to the
converter. It is recommended that C
IN
be larger than 10μF.
The reflected ripple voltage will be smaller with larger C
IN
.
The voltage rating of input capacitor should be larger than
maximum input voltage.
Boost Inductor
A 6.8μH inductor is recommended. The inductor must be
able to handle the following average and peak current:
I
D
–
BOOST MOSFET
Due to the parasitic inductance of the trace, the MOSFET
will experience spikes higher that the output voltage when
the MOSFET turns off. Thus, a MOSFET with enough
voltage margin is needed.
The R
DS(ON)
of the MOSFET is critical for power dissipation
and current limit. A MOSFET with low R
DS(ON)
is desired to
get high efficiency and output current, but very low R
DS(ON)
will reduce the loop stability. A MOSFET with 20m
Ω
to 50m
Ω
R
DS(ON)
is recommended. Some recommended MOSFETs
are shown in Table 2.
Rectifier Diode
A high-speed diode is desired due to the high switching
frequency. Schottky diodes are recommended because of
their fast recovery time and low forward voltage. The rectifier
diode must meet the output current and peak inductor
current requirements.
Output Capacitor
The output capacitor supplies the load directly and reduces
the ripple voltage at the output. Output ripple voltage consists
of two components: the voltage drop due to the inductor ripple
current flowing through the ESR of output capacitor, and the
charging and discharging of the output capacitor.
For low ESR ceramic capacitors, the output ripple is
dominated by the charging and discharging of the output
capacitor. The voltage rating of the output capacitor should
be greater than the maximum output voltage.
D
V
-----------------------
V
–
O
=
(EQ. 7)
TABLE 2. RECOMMENDED MOSFETs
PART
NUMBER
MANUFACTURER
FEATURE
FDC655AN
Fairchild
Semiconductor
6.3A, 30V, R
DS(ON)
= 23m
Ω
FDS4488
Fairchild
Semiconductor
7.9A, 30V, R
DS(ON)
= 22m
Ω
Si7844DP
Vishay
10A, 30V, R
DS(ON)
= 22m
Ω
20A, 30V, R
DS(ON)
= 30m
Ω
SI6928DQ
Vishay
I
LAVG
-------------
=
(EQ. 8)
I
LPK
I
LAVG
Δ
I
L
2
--------
+
=
(EQ. 9)
V
RIPPLE
I
LPK
ESR
V
-----------------------
V
–
O
I
OUT
---------------
1
f
S
----
×
×
+
×
=
(EQ. 10)
ISL97522
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