ISL78022 Intersil Corporation, ISL78022 Datasheet - Page 14

• Manufacturer Part Number: ISL78022
• Description: (ISL78020 / ISL78022) Automotive Grade 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” on page 13), which increases the pole
frequency to: fp = fT*(1+ Hfe *re/R
re = KT/qIc. Thus, choose the lowest value R
design as long as there is still enough base current (I
support the maximum output current (I
We will take as an example the V
Fairchild MMBT3906 PNP transistor is used as the external
pass transistor (Q11 in the “Typical Application Circuit” on
page 18) then for a maximum V
50mA the data sheet indicates Hfe_min = 60. The base-emitter
saturation voltage is: Vbe_max = 0.7V.
For the ISL78020 and ISL78022, the minimum drive current
is shown in Equation 12:
The minimum base-emitter resistor, RBP, can now be
calculated as:
This is the minimum value that can be used; (choose a
convenient value greater than this minimum value, i.e.: 700Ω).
Larger values may be used to reduce quiescent current,
however, regulation may be adversely affected by supply noise
if R
I
(
------------------------------------------------------------ -
DRVP MIN
RBP
BP
I
-------------------------------------------
DRVP MIN
MIN
is made too high in value.
(
Hfe
(
=
)
MIN
VBE
=
)
2mA
–
MAX
Ic
)
=
BE
T
----------------------------------- -
FIGURE 21. THE LINEAR REGULATOR CONTROLS ONE STAGE OF CHARGE PUMP
2mA 50mA
-----------------------------------
/Hfe. Therefore, in order to
(R
BP
0.7V
14
60
–
ON
, R
ON
BL
operating requirement of
BE
linear regulator. If a
ISL78022
, R
T
)/Hfe, where
=
) are usually specified
C
BN
).
600Ω
in the “Functional
DRVP
FBP
BE
in the
0.1µF
ISL78020, ISL78022
(EQ. 12)
(EQ. 13)
B
0.47µF
) to
700Ω
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
stages is determined by the input and output voltage for
positive charge pump stages in Equation 14:
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
Equation 15:
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
A 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 16:
where f
Discontinuous/Continuous Boost Operation and
its Effect on the Charge Pumps
The ISL78020 and ISL78022 V
uses LX switching edges to drive diode charge pumps from
C
0.1µF
POSITIVE
NEGATIVE
OUT
≥
OSC
------------------------------------------------------
2
CE
×
0.1µF
V
V
≥
LX
is the dropout voltage of the pass component of
≥
BOOST
RIPPLE
is the switching frequency.
V
------------------------------------------------------------- -
------------------------------------------------ -
V
OUT
V
I
OUT
OUTPUT
V
INPUT
INPUT
+
×
V
0.22µF
f
CE
–
OSC
(>36V)
–
2
+
V
–
2
F
×
ON
V
V
×
V
is the forward-voltage of the
CE
INPUT
V
F
F
ON
and V
OFF
BOOST
architecture
December 6, 2007
, single or
(EQ. 15)
(EQ. 16)
(EQ. 14)
FN6386.2