ISL78010 Datasheet, PDF (14/18 Pages) Intersil Corporation – Automotive Grade TFT-LCD Power Supply

ISL78010
more output current under the low dropout condition (forced
beta of 10). Typical V_LOGIC voltage supported by the
ISL78010 ranges from +1.3V to V_DD - 0.2V. A fault
comparator is also included for monitoring the output
voltage. The undervoltage threshold is set at 25% below the
1.2V reference.
Set-Up Output Voltage
Refer to the “Typical Application Diagram” on page 17, the
output voltages of V_ON, V_OFF, and V_LOGIC are determined
by Equations 12, 13 and 14:
V_ON
=
V_REF
×
⎛
⎜1
⎝
+
RR_{-----11---21-⎠⎟⎞}
(EQ. 12)
V_OFF
=
V_R
E
F
N
+
R_-----2---2-
R₂₁
×
(
V_REF
N
–
V_R
EF⁾
(EQ. 13)
V_LOGIC
=
V_R
E
F
×
⎛
⎜1
⎝
+
R^{-R----44---21-}_{⎠⎟⎞}
(EQ. 14)
where V_REF = 1.2V, V_REFN = 0.2V.
Resistor networks in the order of 250kΩ, 120kΩ and 10kΩ
are recommended for V_ON, V_OFF and V_LOGIC, respectively.
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:
N_P
O
S
I
T
I
V
E
≥
V_{-----O----U----T-----+-----}V_{----C----E-----–-----}V_{----I--N----P----U----T--}
V_INPUT – 2 × V_F
(EQ. 15)
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. V_F is the forward-voltage of the charge pump
rectifier diode.
The number of negative charge pump stages is given by:
N_NEGATIVE ≥ --^V-_V---^O-_I--_N--^U--_P--^T--_U-^P---_T-^U----–-^T---₂---+--_×---^V--_V--^C--_F--^E--
(EQ. 16)
To achieve high efficiency and low material cost, the lowest
number of charge pump stages which can meet the above
requirements, is always preferred.
High Charge Pump Output Voltage (>36V)
Applications
In the applications where the charge pump output voltage is
over 36V, an external NPN transistor needs to be inserted
between DRVP pin and base of pass transistor Q₃ as shown
in Figure 26; or the linear regulator can control only one
stage charge pump and regulate the final charge pump
output as shown in Figure 27.
CHARGE PUMP
V_IN
OUTPUT
OR A_VDD
7kΩ
DRVP
Q3
NPN
CASCODE
TRANSISTOR
ISL78010
FBP
V_ON
FIGURE 26. CASCODE NPN TRANSISTOR CONFIGURATION
FOR HIGH CHARGE PUMP OUTPUT VOLTAGE
(>36V)
0.1µF
0.1µF
7kΩ
DRVP
Q3
0.1µF
ISL78010
0.47µF
0.1µF
FBP
LX
A_VDD
0.1µF
V_ON
(>36V)
0.22µF
FIGURE 27. THE LINEAR REGULATOR CONTROLS ONE
STAGE OF CHARGE PUMP
Discontinuous/Continuous Boost Operation and
its Effect on the Charge Pumps
The ISL78010 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_OFF supplies. It can
be appreciated that should a regular supply of LX switching
edges be interrupted, for example, during discontinuous
operation at light A_VDD boost load currents, then this may
affect the performance of V_ON and V_OFF regulation -
depending on their exact loading conditions at the time.
To optimize V_ON/V_OFF regulation, the boundary of
discontinuous/continuous operation of the boost converter
can be adjusted, by suitable choice of inductor given V_IN,
V_OUT, switching frequency and the A_VDD current loading, to
be in continuous operation.
14
FN6501.0
May 30, 2007