EL7585 INTERSIL [Intersil Corporation], EL7585 Datasheet - Page 13

• Manufacturer Part Number: EL7585
• Description: TFT-LCD Power Supply
• Manufacturer: INTERSIL [Intersil Corporation]

the transistor. V
rectifier diode.
The number of negative charge pump stages is given by:
N
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.
NEGATIVE
FIGURE 26. CASCODE NPN TRANSISTOR CONFIGURATION
FIGURE 27. THE LINEAR REGULATOR CONTROLS ONE
EL7585
DRVP
EL7585
≥
FBP
------------------------------------------------ -
FOR HIGH CHARGE PUMP OUTPUT VOLTAGE
(>36V)
STAGE OF CHARGE PUMP
V
V
OUTPUT
F
INPUT
0.1µF
is the forward-voltage of the charge pump
DRVP
0.47µF
FBP
7kΩ
–
2
OR A
+
×
TRANSISTOR
V
V
CASCODE
V
CE
Q3
IN
F
VDD
13
0.1µF
NPN
CHARGE PUMP
7kΩ
0.1µF
OUTPUT
0.1µF
Q3
0.1µF
LX
A
VDD
V
ON
0.22µF
(>36V)
V
ON
EL7585
Discontinuous/Continuous Boost Operation and
its Effect on the Charge Pumps
The EL7585 V
edges to drive diode charge pumps from which LDO
regulators generate the V
appreciated that should a regular supply of LX switching
edges be interrupted, for example during discontinuous
operation at light A
affect the performance of V
depending on their exact loading conditions at the time.
To optimize V
discontinuous/continuous operation of the boost converter
can be adjusted, by suitable choice of inductor given V
V
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
where the duty cycle, D = (A
For example, with V
12V we find continuous operation of the boost converter can
be guaranteed for:
L = 10µH and I(A
L = 6.8µH and I(A
L = 3.3µH and I(A
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:
C
where f
Start-Up Sequence
Figure 28 shows a detailed start-up sequence waveform. For
a successful power-up, there should be six peaks at V
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
starts to charge C
ramp followed by a slow ramp. If EN is low at this point, the
C
The first four ramps on C
initialize the fault protection switch and to check whether
there is a fault condition on C
OUT
OUT
DLY
VDD
, switching frequency and the A
≥
ramp will be delayed until EN goes high.
_load) > D*(1-D)*V
OSC
------------------------------------------------------
2
×
V
RIPPLE
is the switching frequency.
ON
ON
I
OUT
DD
/V
VDD
VDD
VDD
DLY
and V
) exceeds 2.5V, an internal current source
VDD
OFF
×
IN
) > 61mA
f
) > 89mA
) > 184mA
OSC
to an upper threshold using a fast
= 5V, F
regulation, the boundary of
boost load currents, then this may
OFF
DLY
ON
IN
ON
/(2*L*F
VDD
architecture uses LX switching
and V
(two up, two down) are used to
DLY
OSC
and V
- V
or V
= 1.0MHz and A
OSC
OFF
IN
OFF
VDD
REF
)/A
)
supplies. It can be
VDD
regulation -
current loading, to
. If a fault is
VDD
July 1, 2005
FN7345.1
CDLY
IN
=
,
.