ADP1882-1.0-EVALZ Analog Devices Inc, ADP1882-1.0-EVALZ Datasheet - Page 26

• Manufacturer Part Number: ADP1882-1.0-EVALZ
• Description: 1 MHz Synchronous Current-Mode Buck Controller Eval. Board
• Manufacturer: Analog Devices Inc

Specifications of ADP1882-1.0-EVALZ

• Silicon Manufacturer: Analog Devices
• Application Sub Type: PWM Buck Controller
• Kit Application Type: Power Management - Voltage Regulator
• Silicon Core Number: ADP1882
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

ADP1882/ADP1883
EFFICIENCY CONSIDERATIONS
One of the important criteria to consider in constructing a dc-to-dc
converter is efficiency. By definition, efficiency is the ratio of the
output power to the input power. For high power applications at
load currents up to 20 A, the following are important MOSFET
parameters that aid in the selection process:
•
•
•
•
•
The following are the losses experienced through the external
component during normal switching operation:
•
•
•
•
•
Channel Conduction Loss
During normal operation, the bulk of the loss in efficiency is
due to the power dissipated through MOSFET channel conduc-
tion. Power loss through the upper-side MOSFET is directly
proportional to the duty cycle (D) for each switching period,
and the power loss through the lower-side MOSFET is directly
proportional to 1 − D for each switching period. The selection
of MOSFETs is governed by the amount of maximum dc load
current that the converter is expected to deliver. In particular,
the selection of the lower-side MOSFET is dictated by the
maximum load current because a typical high current application
employs duty cycles of less than 50%. Therefore, the lower-side
MOSFET is in the on state for most of the switching period.
MOSFET Driver Loss
Other dissipative elements are the MOSFET drivers. The con-
tributing factors are the dc current flowing through the driver
during operation and the Q
where:
V
minus the rectifier drop (see Figure 81)).
C
C
I
V
BIAS
upperFET
lowerFET
DR
DD
is the dc current flowing into the upper and lower-side drivers.
is the driver bias voltage (that is, the low input voltage (V
is the bias voltage.
V
gate and the source
R
conduction
Q
C
C
Channel conduction loss (both MOSFETs)
MOSFET driver loss
MOSFET switching loss
Body diode conduction loss (lower-side MOSFET)
Inductor loss (copper and core loss)
P
P
+
DS (ON)
N1
N2
GS (TH)
G
N1,N2(CL)
DR
[
, the total gate charge
is the input gate capacitance of the lower-side MOSFET.
V
is the input gate capacitance of the upper-side MOSFET.
, the input capacitance of the upper-side switch
, the input capacitance of the lower-side switch
(
LOSS
DD
, the MOSFET support voltage applied between the
, the MOSFET on resistance during channel
×
)
(
=
=
f
[
[
SW
V
D
DR
C
×
lowerFET
R
×
N1(ON)
(
GATE
f
SW
V
C
parameter of the external MOSFETs.
+
DD
upperFET
(
1
+
−
I
D
BIAS
V
)
×
DR
)
R
]
N2(ON)
+
I
BIAS
]
×
)
]
I
2
LOAD
Rev. 0 | Page 26 of 40
DD
)
Switching Loss
The SW node transitions as a result of the switching activities
of the upper-side and lower-side MOSFETs. This causes removal
and replenishing of charge to and from the gate oxide layer of
the MOSFET, as well as to and from the parasitic capacitance that
is associated with the gate oxide edge overlap and the drain and
source terminals. The current that enters and exits these charge
paths presents additional loss during these transition times. This
loss can be approximately quantified by using the following equa-
tion, which represents the time in which charge enters and exits
these capacitive regions:
where:
R
C
The ratio of this time constant to the period of one switching cycle
is the multiplying factor to be used in the following expression:
or
GATE
TOTAL
t
P
Figure 81. Internal Rectifier Voltage Drop vs. Switching Frequency
P
SW-TRANS
is the gate input resistance of the MOSFET.
800
720
640
560
480
400
320
240
160
SW(LOSS)
SW
is the C
80
300
(
LOSS
)
= f
= R
=
GD
400
SW
t
V
V
V
GATE
+ C
SW
DD
DD
DD
× R
t
-
SW
TRANS
= 2.7V
= 3.6V
= 5.5V
GS
× C
SWITCHING FREQUENCY (kHz)
500
GATE
of the external MOSFET used.
TOTAL
×
× C
I
LOAD
600
TOTAL
×
V
× I
700
IN
×
LOAD
2
800
× V
IN
× 2
900
+125°C
+25°C
–40°C
1000