ADP1876-EVALZ AD [Analog Devices], ADP1876-EVALZ Datasheet - Page 21

• Manufacturer Part Number: ADP1876-EVALZ
• Description: 600 kHz Dual Output Synchronous Buck
• Manufacturer: AD [Analog Devices]

Data Sheet
where:
V
and 5 V, depending on the input voltage.
V
this voltage can be estimated by inspecting the gate charge
graph given in the MOSFET data sheet.
R
(listed in Table 1), when charging the MOSFET.
R
(listed in Table 1), when discharging the MOSFET.
R
data sheet. If an external gate resistor is added, add this external
resistance to R
The total power dissipation of the high-side MOSFET is the
sum of conduction and transition losses:
The synchronous rectifier, or low-side MOSFET, carries the
inductor current when the high-side MOSFET is off. The low-
side MOSFET transition loss is small and can be neglected in
the calculation. For high input voltage and low output voltage,
the low-side MOSFET carries the current most of the time.
Therefore, to achieve high efficiency, it is critical to optimize
the low-side MOSFET for low on resistance. In cases where the
power loss exceeds the MOSFET rating or lower resistance is
required than is available in a single MOSFET, connect multiple
low-side MOSFETs in parallel. The equation for low-side
MOSFET conduction power loss is
There is an additional power loss during the time known as
dead time between the turn off of the high-side switch and the
turn on of the low-side switch when the body diode of the low-
side MOSFET conducts the output current.
The power loss in the body diode is given by
where:
V
t
some medium-size MOSFETs with input capacitance, C
approximately 3 nF. The dead time is not fixed. Its effective
value varies with gate drive resistance and C
P
Therefore, the power loss in the low-side MOSFET becomes
D
ON_SOURCE
ON_SINK
GATE
BODYDIODE
DD
SP
F
is the dead time in the ADP1876, typically 30 ns when driving
is the forward voltage drop of the body diode, typically 0.7 V.
is the switching point where the MOSFET fully conducts;
is the input supply voltage to the driver and is between 2.75 V
P
P
I
P
P
is the on gate resistance of MOSFET listed in the MOSFET
BODYDIODE
LS
DRIVER
HS
CLS
is the on resistance of the
= P
≅
is the on resistance of
≅
in high load current designs and low voltage designs.
P
CLS
_
(
C
FALL
I
+
LOAD
GATE
+ P
= V
P
T
≅
BODYDIODE
.
F
)
R
2
× t
ON
×
D
R
_
× f
SINK
DSON
V
SW
SP
+



× I
1
R
GATE
theADP1876
−
O
ADP1876
V
V
OUT
IN



iss
thereby increasing
internal driver
internal driver
iss
, of
Rev. A | Page 21 of 24
Note that MOSFET R
with a typical temperature coefficient of 0.4%/
junction temperature rise over the ambient temperature is
where:
θ
T
P
LOOP COMPENSATION
As with most current mode step-down controllers, a trans-
conductance error amplifier is used to stabilize the external
voltage loop. Compensating the
compensator is needed between COMP and AGND. Figure 31
shows the configuration of the compensation components:
R
C
compensation analysis.
The open-loop gain transfer function at angular frequency, s, is
given by
where:
g
G
Z
Z
V
G
where:
A
24 V/V set by the gain resistor between DLx and PGNDx.
R
If a sense resistor, R
then G
m
JA
D
A
COMP
COMP
FILTER
DSON_MIN
COMP
CS
REF
CS
CS
is the transconductance of the error amplifer, 500 µS
is the ambient temperature.
is the total power dissipated in the MOSFET.
is the thermal resistance of the MOSFET package.
is the tranconductance of the current sense amplifier.
is the current sense gain of either 3 V/V, 6 V/V, 12 V/V, or
with units of A/V is given by
T
= 0.6 V
G
G
H
, C
, to simplify calculation, C
J
is the impedance of the compensation network.
is the impedance of the output filter.
CS
CS
CS
= T
) (
s
COMP
becomes
is the the low-side MOSFET minimum on resistance.
=
=
=
A
A
A
+ θ
, and C
g
CS
CS
m
C
C2
JA
×
Figure 31. Compensation Components
×
×
(
× P
G
R
R
CS
S
C2
1
DSON
DSON
, is added in series with the low-side FET,
D
DSON
. Because C
×
COMPx
R
1
C
AGND
COMP
COMP
V
V
_
_
OUT
MIN
MIN
REF
increases as temperature increases
+
×
R
ADP1876
C2
Z
ADP1876
C2
S
COMP
)
is ignored for the stability
0.6V
is very small compared to
g
m
) (
s
×
is fairly easy; an RC
Z
FBx
FILTER
o
C. The MOSFET
ADP1876
) (
s
(1)
(2)