Applications information – Rainbow Electronics MAX17117 User Manual
Page 20
Internal-Switch Boost Regulator with Integrated
7-Channel Scan Driver, Op Amp, and LDO
MAX17117
20 _____________________________________________________________________________________
Operational Amplifier Output Voltage
Using the buffer configuration as shown in Figure 1, the
output voltage of the operational amplifier is adjusted by
connecting a resistive voltage-divider from the output
(V
MAIN
) to AGND with the center tap connected to POS
(see Figure 1). Select R3 in the 10kI to 100kI range.
Calculate R4 with the following equation:
MAIN
OUT
V
R3 R4
1
V
=
× −
Place R3 and R4 close to the IC such that the connec-
tions between these components and the POS pin are
kept as short as possible.
LDO Output Voltage
The output voltage of the LDO is adjusted by connect-
ing a resistive voltage-divider from the output (V
LDOO
)
to AGND with the center tap connected to LDOADJ
(see Figure 1). Select R6 in the 10kI to 50kI range.
Calculate R5 with the following equation:
LDOO
V
R5 R6
1
1.24V
=
×
−
Place R5 and R6 close to the IC such that the connec-
tions between these components and the LDOADJ pin
are kept as short as possible.
Connect a 1FF low ESR capacitor between LDOO and
AGND to ensure stability and to provide good output-
transient performance.
Scan Driver
Setting the Gate-Shading Period Time Duration
To set the gate-shading period time duration, configure
R
SET
and C
SET
as shown in Figure 1. Choose a C
SET
value greater than 35pF, then calculate the required
R
SET
value that gives the desired gate-shading period
time duration with the following equation:
SET
SET
LDOO
t
R
1.24V
ln 1
C
V
−
=
−
×
Increase or decrease C
SET
as needed and repeat the
above calculation to achieve the desired gate-shading
period time duration, while ensuring C
SET
remains great-
er than 35pF and R
SET
is within the 8kI to 100kI range.
Place R
SET
and C
SET
close to the IC such that the con-
nections between these components and the DTS pin
are kept as short as possible.
Gate-Shading Discharge Resistors
For proper operation, choose R
O
and R
E
discharge
resistors that are greater than 100I. Place R
O
and R
E
close to the IC such that the connections between these
components and their respective pins are kept as short
as possible.
Applications Information
Power Dissipation
An IC’s maximum power dissipation depends on the
thermal resistance from the die to the ambient environ-
ment and the ambient temperature. The thermal resis-
tance depends on the IC package, PCB copper area,
other thermal mass, and airflow.
The MAX17117, with its exposed backside paddle sol-
dered to 1in
2
of PCB copper, can dissipate approximate-
ly 1990mW into +70NC still air. More PCB copper, cooler
ambient air, and more airflow increase the possible
dissipation, while less copper or warmer air decreases
the IC’s dissipation capability. The major components of
power dissipation are the power dissipated in the step-
up regulator and the power dissipated by the operational
amplifiers.
The MAX17117’s largest on-chip power dissipation
occurs in the step-up switch, the VCOM amplifier, the
CKH_ level shifters, and the LDO.
Step-Up Regulator
The largest portions of the power dissipated by the
step-up regulator are the internal MOSFET, the induc-
tor, and the output diode. If the step-up regulator with
3.3V input and 285mA output has approximately 85%
efficiency, approximately 5% of the power is lost in the
internal MOSFET, approximately 3% in the inductor, and
approximately 5% in the output diode. The remaining
few percent are distributed among the input and out-
put capacitors and the PCB traces. If the input power
is approximately 2.85W, the power lost in the internal
MOSFET is approximately 143mW.
Operational Amplifier
The power dissipated in the operational amplifier
depends on the output current, the output voltage, and
the supply voltage:
(
)
SOURCE
VCOM_SOURCE
AVDD
VCOM
PD
I
V
V
=
×
−
SINK
VCOM_SINK
VCOM
PD
I
V
=
×
where I
VCOM
_
SOURCE
is the output current sourced by
the operational amplifier, and I
VCOM
_
SINK
is the output
current that the operational amplifier sinks. In a typical