Datasheet

MAX17116
Dual-Output DC/DC
Power Supply for AMOLED
17
The conduction power loss in the synchronous rectifier is:
SYN1(RMS)
2
SYN1 DSON_SYN1
P = I R ×
where:
LP_RIPPLE
2
SYN1(RMS) LP(DC_MAX) 1
LP(DC_MAX)
I
1
I I (1- D ) [1 ( ) ]
3 I
∆
= × × + ×
R
DSON_SYN1
is the on-resistance for the synchronous
rectifier.
D
1
is the duty cycle on the step-up regulator.
In general, at full power, if the switch sizes have been
chosen well, the switching losses in the main switch are
approximately equal to the conduction losses in that
switch. The synchronous rectifier switching losses are
small since switching occurs with zero voltage across
the switch.
Inverting Converter
The largest portions of power dissipation in the invert-
ing regulator are the internal MOSFET, the inductor,
and the synchronous rectifier. If the inverting regulator
has 85% efficiency, approximately 7% to 10% of the
power is lost in the internal MOSFET and synchronous
rectifier, approximately 4% to 5% in the inductor. The
remaining 1% to 4% is distributed among the input and
output capacitors and the PCB traces. This gives a good
estimate of the power dissipated in the IC by the invert-
ing regulator. Like the step-up converter, the following
formulas can be used to estimate the power loss in the
internal power MOSFET and the synchronous rectifier
(excluding switching losses):
The conduction power loss in the internal power MOSFET is:
SW2(RMS)
2
LXN_ON DSON_LXN
P = I R×
where:
LN_RIPPLE
2
SYN2(RMS) LN(DC_MAX) 2
LN(DC_MAX)
I
1
I I (1- D ) [1 ( ) ]
3 I
∆
= × × + ×
R
DSON_LXN
is the on-resistance for the internal power
MOSFET.
D
2
is the duty cycle on the inverting converter.
The conduction power loss in the synchronous rectifier is:
SYN2(RMS)
2
SYN2 DSON_SYN2
P = I R×
where:
LN_RIPPLE
2
SYN2(RMS) LN(DC_MAX) 2
LN(DC_MAX)
I
1
I I (1- D ) [1 ( ) ]
3 I
∆
= × × + ×
R
DSON_SYN2
is the on-resistance for the synchronous
rectifier.
D
2
is the duty cycle on the inverting converter.
In general, at full power, if the switch sizes have been
chosen well, the switching losses in the main switch are
approximately equal to the conduction losses in that
switch. The synchronous rectifier switching losses are
small since switching occurs with zero voltage across
the switch.
PCB Layout and Grounding
Careful PCB layout is important for proper operation.
Use the following guidelines for good PCB layout:
1) Place the input capacitors as close as possible to
the IN pin so the trace connecting one end of the
capacitors to the IN pin and the trace connecting
the other end of the capacitors to the PGND pin is
as short as possible.
2) Place the step-up converter inductor so the traces
connecting the inductor to the LXP pin and the input
capacitors are as short as possible.
3) Connect the output capacitors of OUTP and OUTN
as close as possible to their respective pins.
4) Create a power ground plane (PGND) so the other
end of these capacitors and the PGND pin can con-
nect to this plane directly.
5) Create an analog ground plane (AGND) so the other
end of these capacitors and the AGND pin can con-
nect to this plane directly. Place the inverting con-
verter inductor so the trace connecting the inductor
to the LXN pin and the distance the inductor current
has to travel through the PGND plane to the PGND
pin are as short as possible.