ADP3178JR AD [Analog Devices], ADP3178JR Datasheet - Page 9

• Manufacturer Part Number: ADP3178JR
• Description: 4-Bit Programmable Synchronous Buck Controllers
• Manufacturer: AD [Analog Devices]

R
The value of R
current. The current comparators of the ADP3158 and ADP3178
have a minimum current limit threshold of 69 mV. Note that the
69 mV value cannot be used for the maximum specified nominal
current, as headroom is needed for ripple current and tolerances.
The current comparator threshold sets the peak of the inductor
current yielding a maximum output current, I
the peak inductor current value less half of the peak-to-peak induc-
tor ripple current. From this the maximum value of R
calculated as:
In this case, 4 m was chosen as the closest standard value.
Once R
where current limit is reached, I
the maximum current sense threshold of 87 mV:
At output voltages below 450 mV, the current sense threshold is
reduced to 54 mV, and the ripple current is negligible. There-
fore, at dead short the output current is reduced to:
To safely carry the current under maximum load conditions, the
sense resistor must have a power rating of at least:
Power MOSFETs
Two external N-channel power MOSFETs must be selected for
use with the ADP3158 and ADP3178, one for the main switch
and an identical one for the synchronous switch. The main
selection parameters for the power MOSFETs are the threshold
voltage (V
The minimum input voltage dictates whether standard threshold
or logic-level threshold MOSFETs must be used. For V
standard threshold MOSFETs (V
SENSE
I
R
OUT CL
SENSE
P
I
OUT SC
R
(
SENSE
SENSE
(
87
GS(TH)
)
4
)
I
m
mV
has been chosen, the output current at the point
V
V
O
(
SENSE
I
CS CL
CS CL
) and the ON-resistance (R
O
54
R
4
(
(
)
–
I
SENSE
2
L RIPPLE
m
mV
is based on the maximum required output
(
3 8
)(
)(
.
MAX
R
MIN
2
2
SENSE
A
)
)
13 5
)
–
20
.
I
L RIPPLE
15
(
(
A
OUT(CL)
20
A
2
GS(TH)
69
A
A
mV
)
1 9
)
2
, can be calculated using
.
< 4 V) may be used. If
4
A
DS(ON)
O
m
, which equals twice
4
).
m
1 6
SENSE
.
W
IN
> 8 V,
is
(11)
(10)
(8)
(9)
V
(V
MOSFETs with V
value of V
The maximum output current I
requirement for the two power MOSFETs. When the ADP3158
and ADP3178 are operating in continuous mode, the simplifying
assumption can be made that one of the two MOSFETs is always
conducting the average load current. For V
1.65 V, the maximum duty ratio of the high-side FET is:
The maximum duty ratio of the low-side (synchronous rectifier)
MOSFET is:
The maximum rms current of the high-side MOSFET is:
The maximum rms current of the low-side MOSFET is:
The R
dissipation. If 10% of the maximum output power is allowed for
MOSFET dissipation, the total dissipation will be:
Allocating half of the total dissipation for the high-side MOSFET
and half for the low-side MOSFET and assuming that switching
losses are small relative to the dc conduction losses, the required
minimum MOSFET resistances will be:
I
I
I
I
RMSHSF
RMSHSF
RMSLSF
RMSLSF
IN
GS(TH)
is expected to drop below 8 V, logic-level threshold MOSFETs
D
D
D
R
R
P
DS(ON)
D FETs
DS ON HSF
DS ON LSF
HSF MAX
HSF MAX
LSF MAX
(
(
(
< 2.5 V) are strongly recommended. Only logic-level
(
(
(
CC
D
D
54
36
HSF MAX
LSF MAX
)
)
)
%
%
for each MOSFET can be derived from the allowable
should be used.
(
(
)
)
)
0 1
13 1
13 1
.
1
)
1
1
)
.
.
GS
I
I
P
P
–
– (
– (
LSF
A
A
HSF
V
LSF
I
HSF
I
ratings higher than the absolute maximum
D
2
L VALLEY
L VALLEY
2
OUT
195
(
(
f
HSF MAX
2
MIN
2
(
(
13 1
13 1
(
kHz
.
1 13
10 8
.
1 13
I
)
)
.
8 8
2
2
.
A
A
OUT MAX
t
.
ADP3158/ADP3178
O(MAX)
.
OFF
3
3
)
(
I
W
16 1
A
I
16 1
L VALLEY
W
A
(
L VALLEY
3 3
(
)
(
2
.
2
.
.
54
A
A
determines the R
%
)
)
)
10
s
3
15
3
)
)
)
16 1
16 1
IN
2 26
m
m
I
.
.
I
.
L PEAK
36
= 5 V and V
L PEAK
(
(
A
A
%
2
W
2
)
)
)
8 8
10 8
.
I
L PEAK
I
.
L PEAK
(
A rms
DS(ON)
(
A rms
OUT
)
2
)
2
=
(12)
(13)
(14)
(15)
(16)
(17)
(18)