ISL97642 Intersil Corporation, ISL97642 Datasheet - Page 14

• Manufacturer Part Number: ISL97642
• Description: TFT-LCD DC/DC
• Manufacturer: Intersil Corporation

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Calculation of the Linear Regulator Base-emitter
Resistors (RBP and RBN)
For the pass transistor of the linear regulator, low frequency
gain (Hfe) and unity gain frequency (f
in the datasheet. The pass transistor adds a pole to the loop
transfer function at fp = f
maintain phase margin at low frequency, the best choice for
a pass device is often a high frequency, low gain switching
transistor. Further improvement can be obtained by adding a
base-emitter resistor R
Block Diagram), which increases the pole frequency to:
fp = fT*(1+ Hfe *re/R
the lowest value R
enough base current (I
current (I
We will take as an example the V
Fairchild MMBT3906 PNP transistor is used as the external
pass transistor (Q11 in the application diagram), then for a
maximum V
sheet indicates Hfe_min = 60. The base-emitter saturation
voltage is: Vbe_max = 0.7V.
For the ISL97642, the minimum drive current is:
I_DRVP_min = 2mA
The minimum base-emitter resistor, R
calculated as:
This is the minimum value that can be used – so, we now
choose a convenient value greater than this minimum value;
for example, 700Ω. Larger values may be used to reduce
quiescent current, however, regulation may be adversely
affected by supply noise if R
RBP_min
(
(
0.7V
)
⁄
(
C
=
2mA
).
VBE_max (I_DRVP_min - Ic/Hfe_min =
ON
–
operating requirement of 50mA, the data
(
50mA
BE
BE
⁄
in the design as long as there is still
) 60
B
BE
)/Hfe, where re = KT/qIc. So choose
⁄
) to support the maximum output
T
FIGURE 21. THE LINEAR REGULATOR CONTROLS ONE STAGE OF CHARGE PUMP
/Hfe. Therefore, in order to
(R
)
)
=
BP
BP
14
600Ω
, R
is made too high in value.
ON
BL
ISL97642
, R
T
linear regulator. If a
BP
) are usually specified
BN
, can now be
in the Functional
DRVP
FBP
0.1µF
(EQ. 11)
0.47µF
700Ω
ISL97642
Q11
0.1µF
0.1µF
Charge Pump
To generate an output voltage higher than V
multiple stages of charge pumps are needed. The number of
stage is determined by the input and output voltage. For
positive charge pump stages:
N
where V
the linear regulator. It ranges from 0.3V to 1V depending on
the transistor selected. V
charge-pump 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.
Charge Pump Output Capacitors
Ceramic capacitor with low ESR is recommended. With
ceramic capacitors, the output ripple voltage is dominated by
the capacitance value. The capacitance value can be
chosen by Equation 14:
where f
Discontinuous/Continuous Boost Operation and
its Effect on the Charge Pumps
The ISL97642 V
switching edges to drive diode charge pumps from which
LDO regulators generate the V
be appreciated that should a regular supply of LX switching
C
0.1µF
POSITIVE
NEGATIVE
OUT
≥
OSC
------------------------------------------------------
2
CE
×
0.1µF
V
V
≥
is the dropout voltage of the pass component of
LX
≥
BOOST
RIPPLE
is the switching frequency.
V
------------------------------------------------------------- -
------------------------------------------------ -
V
OUT
V
I
OUT
OUTPUT
V
INPUT
ON
INPUT
+
×
V
and V
0.22µF
f
CE
–
OSC
–
(>36V)
2
+
V
–
2
F
×
ON
V
V
×
OFF
V
is the forward-voltage of the
CE
INPUT
V
F
F
ON
architecture uses LX
and V
OFF
BOOST
supplies. It can
, single or
June 18, 2007
(EQ. 14)
(EQ. 12)
(EQ. 13)
FN6436.0