MSS1048-152NL AD [Analog Devices], MSS1048-152NL Datasheet - Page 22

• Manufacturer Part Number: MSS1048-152NL
• Description: Dual 5 A, 20 V Synchronous Step-Down
• Manufacturer: AD [Analog Devices]

LOW-SIDE POWER DEVICE SELECTION
The
can drive the low-side N-channel MOSFETs (NFETs). The selec-
tion of the low-side N-channel MOSFET affects the dc-to-dc
regulator performance.
The selected MOSFET must meet the following requirements:
•
•
The
the selected MOSFET can be fully turned on at 5 V.
Total gate charge (Qg at 5 V) must be less than 50 nC. Lower Qg
characteristics constitute higher efficiency.
When the high-side MOSFET is turned off, the low-side MOSFET
carries the inductor current. For low duty cycle applications, the
low-side MOSFET carries the current for most of the period. To
achieve higher efficiency, it is important to select a low on-resist-
ance MOSFET. The power conduction loss for the low-side
MOSFET can be calculated by
where R
Make sure that the MOSFET can handle the thermal dissipation
due to the power loss.
In some cases, efficiency is not critical for the system; therefore,
the diode can be selected as the low-side power device. The
average current of the diode can be calculated by
The reverse breakdown voltage rating of the diode must be
greater than the input voltage with an appropriate margin to
allow for ringing, which may be present at the SWx node. A
Schottky diode is recommended because it has a low forward
voltage drop and a fast switching speed.
If a diode is used for the low-side device, the
enable the PFM mode by connecting the MODE pin to ground.
Table 10. Recommended MOSFETs
Vendor
Fairchild
Fairchild
Fairchild
Vishay
Vishay
AOS
AOS
ADP2325
ADP2325
ADP2325
Drain source voltage (V
Drain current (I
I
P
I
LIMIT_MAX
DIODE (AVG)
FET_LOW
DSON
Part No.
FDS8880
FDMS7578
FDS6898A
Si4804CDY
SiA430DJ
AON7402
AO4884L
is the on resistance of the low-side MOSFET.
= I
is the selected maximum current-limit threshold.
has integrated low-side MOSFET drivers, which
low-side gate drive voltage is 5 V. Make sure that
= (1 − D) × I
OUT
2
D
× R
) must be greater than 1.2 × I
DSON
V
30 V
25 V
20 V
30 V
20 V
30 V
40 V
DS
OUT
× (1 − D)
DS
) must be higher than 1.2 × V
I
10.7 A
14 A
9.4 A
7.9 A
10.8 A
39 A
10 A
D
R
12 mΩ
8 mΩ
14 mΩ
27 mΩ
18.5 mΩ
15 mΩ
16 mΩ
ADP2325
DSON
LIMIT_MAX
must
Qg
12 nC
8 nC
16 nC
7 nC
5.3 nC
7.1 nC
13.6 nC
, where
IN
.
Rev. 0 | Page 22 of 32
PROGRAMMING THE UVLO INPUT
The precision enable input can be used to program the UVLO
threshold and hysteresis of the input voltage, as shown in Figure 50.
Use the following equation to calculate R
where:
V
V
COMPENSATION COMPONENTS DESIGN
In peak current mode control, the power stage can be simplified
to a voltage controlled current source supplying current to the
output capacitor and load resistor. It is composed of one domain
pole and a zero contributed by the output capacitor ESR. The
control-to-output transfer function is shown in the following
equations:
where:
A
R is the load resistance.
C
R
OUT
ESR
IN_RISING
IN_FALLING
VI
= 8.33 A/V.
is the equivalent series resistance of the output capacitor.
G
is the output capacitance.
R
R
f
f
P
Z
TOP_EN
VD
BOT
=
=
is the V
(
_
is the V
s
2
2
)
EN
R
R
×
×
=
TOP_EN
BOT_EN
=
π
π
=
V
V
Figure 50. Programming the UVLO Input
1
×
COMP
×
V
1 .
IN
OUT
(
R
IN
IN
R
V
rising threshold.
ESR
PVINx
1
_
falling threshold.
(
1
+
×
(
RISING
s
ENx
1 .
s
)
1
R
V
)
×
V
ESR
IN_RISING
=
C
1
×
OUT
A
1µA
)
−
2 .
5
VI
×
R
μA
V
C
TOP
×
4µA
×
OUT
R
−
−
R
_
×
1
EN
1
TOP
2 .








2 .
1
1
V
×
_
1.2V
V
+
+
5
EN
×
2
2
×
μ
V
TOP_EN
×
×
1
Α
IN_FALLING
EN CMP
π
π
μA
−
s
s
×
×
1
Data Sheet
2 .
and R
f
f
Z
P
V








BOT_EN
: