參數(shù)資料
型號： EL7585IL-T7
廠商： INTERSIL CORP
元件分類： 穩(wěn)壓器
英文描述： TFT-LCD Power Supply
中文描述： 3.5 A SWITCHING REGULATOR, 1100 kHz SWITCHING FREQ-MAX, QCC20
封裝： 4 X 4 MM, MO-220, QFN-20
文件頁數(shù)： 13/18頁
文件大小： 732K
代理商： EL7585IL-T7
13
FN7345.1
July 1, 2005
the transistor. V
F
is the forward-voltage of the charge pump
rectifier diode.
The number of negative charge pump stages is given by:
To achieve high efficiency and low material cost, the lowest
number of charge pump stages which can meet the above
requirements, is always preferred.
High Charge Pump Output Voltage (>36V)
Applications
In the applications where the charge pump output voltage is
over 36V, an external npn transistor need to be inserted into
between DRVP pin and base of pass transistor Q3 as shown
in Figure 26; or the linear regulator can control only one
stage charge pump and regulate the final charge pump
output as shown in Figure 27.
Discontinuous/Continuous Boost Operation and
its Effect on the Charge Pumps
The EL7585 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
Charge Pump Output Capacitors
Ceramic capacitors with low ESR are recommended. With
ceramic capacitors, the output ripple voltage is dominated by
the capacitance value. The capacitance value can be
chosen by the following equation:
I
RIPPLE
OSC
where f
OSC
is the switching frequency.
Start-Up Sequence
Figure 28 shows a detailed start-up sequence waveform. For
a successful power-up, there should be six peaks at V
CDLY
.
When a fault is detected, the device will latch off until either
EN is toggled or the input supply is recycled.
If EN is L, the device is powered down. If EN is H, and the
input voltage (V
DD
) exceeds 2.5V, an internal current source
starts to charge C
DLY
to an upper threshold using a fast
ramp followed by a slow ramp. If EN is low at this point, the
C
DLY
ramp will be delayed until EN goes high.
The first four ramps on C
DLY
(two up, two down) are used to
initialize the fault protection switch and to check whether
there is a fault condition on C
DLY
or V
REF
. If a fault is
N
NEGATIVE
V
INPUT
V
F
+
------------------------–
≥
V
IN
OR A
VDD
CHARGE PUMP
OUTPUT
7k
Q3
FBP
EL7585
DRVP
NPN
CASCODE
TRANSISTOR
V
ON
FIGURE 26. CASCODE NPN TRANSISTOR CONFIGURATION
FOR HIGH CHARGE PUMP OUTPUT VOLTAGE
(>36V)
V
ON
(>36V)
0.1μF
0.1μF
0.1μF
0.1μF
7k
.47μF
0.22μF
0.1μF
A
VDD
LX
Q3
FBP
EL7585
DRVP
FIGURE 27. THE LINEAR REGULATOR CONTROLS ONE
STAGE OF CHARGE PUMP
C
OUT
------------------------------------------------------
≥
EL7585
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