el7641 Intersil Corporation, el7641 Datasheet - Page 14

• Manufacturer Part Number: el7641
• Description: Tft-lcd Dc/dc With Integrated Amplifiers
• Manufacturer: Intersil Corporation

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 EL7640, EL7641 and EL7642, the minimum drive
current is:
I_DRVP_min = 2mA
The minimum base-emitter resistor, RBP, can now be
calculated as:
RBP_min = VBE_max/(I_DRVP_min - Ic/Hfe_min) =
0.7V/(2mA - 50mA/60) = 600Ω
This is the minimum value that can be used – so, we now
choose a convenient value greater than this minimum value;
say 700Ω. Larger values may be used to reduce quiescent
current, however, regulation may be adversely affected by
supply noise if R
C
).
ON
operating requirement of 50mA the data
BP
BE
is made too high in value.
BE
in the design as long as there is still
B
BE
)/Hfe, where re = KT/qIc. So choose
) to support the maximum output
T
FIGURE 22. THE LINEAR REGULATOR CONTROLS ONE STAGE OF CHARGE PUMP
/Hfe. Therefore, in order to
(R
BP
14
, R
ON
BL
EL7642
, R
T
linear regulator. If a
) are usually specified
BN
in the Functional
DRVP
FBP
EL7640, EL7641, EL7642
0.1µF
0.47µF
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
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 the following equation:
where f
Discontinuous/Continuous Boost Operation and
its Effect on the Charge Pumps
The EL7640, EL7641 and EL7642 V
architecture uses LX switching edges to drive diode charge
pumps from which LDO regulators generate the V
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
BOOST
OFF
February 22, 2006
ON
, single or
and
FN7415.2