ISL78010_11 Datasheet, PDF (14/19 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
As shown in the “Typical Application Diagram” on page 18,
the output voltages of V_ON, V_OFF, and V_LOGIC are as
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 and 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. Use
Equation 15 to calculate 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
Equation 16:
N_N
E
G
A
T
I
V
E
≥
--^V----^O-----^U----^T---^P----^U-----^T------+-----^V----^C----^E--
V_INPUT – 2 × V_F
(EQ. 16)
To achieve high efficiency and low material cost, the lowest
number of charge pump stages that can meet the above
requirements is preferred.
High Charge Pump Output Voltage (>36V)
Applications
In applications where the charge pump output voltage is over
36V, an external NPN transistor must be inserted between
the DRVP pin and the 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.
Should a regular supply of LX switching edges be
interrupted - for example, during discontinuous operation at
light A_VDD boost load currents - it 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
FN6501.1
May 3, 2011