ISL78022_14 Datasheet, PDF (14/19 Pages) Intersil Corporation – Automotive Grade TFT-LCD DC/DC with Integrated Amplifiers

ISL78020, ISL78022
0.1µF
0.1µF
LX
V_BOOST
DRVP
700Ω
Q11
0.1µF 0.1µF
ISL78022
0.47µF
0.1µF
V_ON
(>36V)
FBP
0.22µF
FIGURE 21. THE LINEAR REGULATOR CONTROLS ONE STAGE OF CHARGE PUMP
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 (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
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” on page 13), which increases the pole
frequency to: fp = fT*(1+ Hfe *re/R_BE)/Hfe, where
re = KT/qIc. Thus, 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 “Typical Application Circuit” on
page 18) 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 ISL78020 and ISL78022, the minimum drive current
is shown in Equation 12:
I_DRVP(MIN) = 2mA
(EQ. 12)
The minimum base-emitter resistor, RBP, can now be
calculated as:
-⁽--^R-----^B----^P----^M-----^I--^N------=-----^V-----^B----^E----^M-----^A----^X----⁾
-^I-^D-----^R----^V----^P----⁽--^M----^I--^N-----⁾---–----^I--^c--
Hfe_MIN
=
------------⁰----^.-⁷----^V--------------
2_----m_------A_{-----–-----}5_---0_----m_-----A_---
60
=
600Ω
(EQ. 13)
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_BP is made too high in value.
14
Charge Pump
To generate an output voltage higher than V_BOOST, single or
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_P
O
S
IT
IV
E
≥
-^V----^O----^U----^T-----+-----^V----^C----^E-----–-----^V----^I--^N----^P----^U----^T--
V_INPUT – 2 × V_F
(EQ. 14)
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
Equation 15:
N_NEGATIVE ≥ --^V-_V---^O-_I--_N--^U--_P--^T--_U-^P---_T-^U----–-^T---₂---+--_×---^V--_V--^C--_F--^E--
(EQ. 15)
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:
C_O
U
T
≥
---------------------^I--^O----^U-----^T----------------------
2 × V_RIPPLE × f_OSC
(EQ. 16)
where f_OSC is the switching frequency.
Discontinuous/Continuous Boost Operation and
its Effect on the Charge Pumps
The ISL78020 and ISL78022 V_ON and V_OFF architecture
uses LX switching edges to drive diode charge pumps from
FN6386.3
December 23, 2013