EL7640, EL7641, EL7642
LX
0.1µF
V
BOOST
0.1µF
700Ω
0.1µF 0.1µF
DRVP
FBP
Q11
V
ON
(>36V)
0.47µF
0.1µF
EL7642
0.22µF
FIGURE 22. THE LINEAR REGULATOR CONTROLS ONE STAGE OF CHARGE PUMP
Calculation of the Linear Regulator Base-emitter
Charge Pump
Resistors (RBP and RBN)
To generate an output voltage higher than V
, single or
BOOST
For the pass transistor of the linear regulator, low frequency
gain (Hfe) and unity gain frequency (fT) are usually specified
in the datasheet. The pass transistor adds a pole to the loop
transfer function at fp = fT/Hfe. Therefore, in order to
multiple stages of charge pumps are needed. The number of
stage is determined by the input and output voltage. For
positive charge pump stages:
V
+ V
– V
OUT
V
CE INPUT
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
-------------------------------------------------------------
≥
N
POSITIVE
– 2 × V
F
INPUT
where V
CE
is the dropout voltage of the pass component of
base-emitter resistor R (R , R , R
in the Functional
Block Diagram), which increases the pole frequency to:
BE BP BL BN
the linear regulator. It ranges from 0.3V to 1V depending on
the transistor selected. V is the forward-voltage of the
charge-pump rectifier diode.
F
fp = fT*(1+ Hfe *re/R )/Hfe, where re = KT/qIc. So choose
BE
the lowest value R in the design as long as there is still
BE
enough base current (I ) to support the maximum output
B
The number of negative charge-pump stages is given by:
current (I ).
C
V
+ V
OUTPUT
CE
------------------------------------------------
≥
N
NEGATIVE
V
– 2 × V
F
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
linear regulator. If a
ON
INPUT
To achieve high efficiency and low material cost, the lowest
number of charge-pump stages, which can meet the above
requirements, is always preferred.
maximum V
operating requirement of 50mA the data
ON
sheet indicates Hfe_min = 60. The base-emitter saturation
voltage is: Vbe_max = 0.7V.
Charge Pump Output Capacitors
For the EL7540, EL7541 and EL7542, the minimum drive
current is:
I_DRVP_min = 2mA
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:
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
I
OUT
------------------------------------------------------
C
≥
OUT
2 × V
× f
OSC
RIPPLE
where f
is the switching frequency.
OSC
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
Discontinuous/Continuous Boost Operation and
its Effect on the Charge Pumps
The EL7640, EL7641 and EL7642 V
and V
ON
OFF
supply noise if R is made too high in value.
BP
architecture uses LX switching edges to drive diode charge
pumps from which LDO regulators generate the V
and
ON
FN7415.1
14
September 26, 2005
INTERSIL [ Intersil ]
