14
FN7415.2
February 22, 2006
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
T) are usually specified
in the datasheet. The pass transistor adds a pole to the loop
transfer function at fp = f
T/Hfe. Therefore, in order to
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
BE
(R
BP
, R
BL
, R
BN
in the Functional
Block Diagram), which increases the pole frequency to:
fp = fT*(1+ Hfe *re/R
BE
)/Hfe, where re = KT/qIc. So choose
the lowest value R
BE
in the design as long as there is still
enough base current (I
B
) to support the maximum output
current (I
C
).
We will take as an example the V
ON
linear regulator. If a
Fairchild MMBT3906 PNP transistor is used as the external
pass transistor, Q11 in the application diagram, then for a
maximum V
ON
operating requirement of 50mA the data
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) = 600
This is the minimum value that can be used – so, we now
choose a convenient value greater than this minimum value;
say 700. Larger values may be used to reduce quiescent
current, however, regulation may be adversely affected by
supply noise if R
BP
is made too high in value.
Charge Pump
To generate an output voltage higher than V
BOOST
, single or
multiple stages of charge pumps are needed. The number of
stage is determined by the input and output voltage. For
positive charge pump stages:
where V
CE
is the dropout voltage of the pass component of
the linear regulator. It ranges from 0.3V to 1V depending on
the transistor selected. 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.
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
OSC
is the switching frequency.
Discontinuous/Continuous Boost Operation and
its Effect on the Charge Pumps
The EL7640, EL7641 and EL7642 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
ON
(>36V)
0.1µF
0.1µF
0.1µF
0.1µF
0.47µF
0.22µF
700
0.1µF
V
BOOST
LX
Q11
FBP
DRVP
EL7642
FIGURE 22. THE LINEAR REGULATOR CONTROLS ONE STAGE OF CHARGE PUMP
N
POSITIVE
V
OUT
V
CE
V
INPUT
–+
V
INPUT
2V
F
–
------------------------------------------------------------- -
N
NEGATIVE
V
OUTPUT
V
CE
+
V
INPUT
2V
F
–
-------------------------------------------------
C
OUT
I
OUT
2V
RIPPLE
f
OSC
------------------------------------------------------
EL7640, EL7641, EL7642
