ADP3178JR AD [Analog Devices], ADP3178JR Datasheet - Page 12

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

ADP3158/ADP3178
Trade-Offs Between DC Load Regulation and AC Load
Regulation
Casual observation of the circuit operation—e.g., with a voltmeter
—would make it appear that the dc load regulation appears to
be rather poor compared to a conventional regulator (see Figure
4). This would be especially noticeable under very light or very
heavy loads where the voltage is “positioned” near one of the
extremes of the regulation window rather than near the nominal
center value. It must be noted and understood that this low gain
characteristic (i.e., loose dc load regulation) is inherently required
to allow improved transient containment (i.e., to achieve tighter
ac load regulation). That is, the dc load regulation is intentionally
sacrificed (but kept within specification) in order to minimize
the number of capacitors required to contain the load transients
produced by the CPU.
Linear Regulators
The two linear regulators provide a low cost, convenient, and
versatile solution for generating additional supply rails. The
maximum output load current is determined by the size and
thermal impedance of the external N-channel power MOSFET
that is placed in series with the supply. The output voltage is
sensed at the LRFB pin and compared to an internal reference
voltage in a negative feedback loop which keeps the output voltage
in regulation. If the load is reduced or increased, the MOSFET
drive will also be reduced or increased by the controller IC to
provide a well-regulated 2.5% accurate output voltage.
The LRFB threshold of the ADP3158 are internally set at 2.5 V
(LRFB1) and 1.8 V (LRFB2), while the LRFB pins of the
ADP3178 are compared to an internal 1 V reference. This allows
the use of an external resistor divider network to program the linear
regulator output voltage. The correct resistor values for setting the
output voltage of the linear regulators in the ADP3178 can be
determined using:
Assuming that R
Equation 32 to solve for R
2.5V, 2.2A
V
R
R
V
OUT LR
U
U
LR2
(
10
10
)
k
k
1 F
100 F
250m
V
L
R
LRFB
= 10 k , V
S
1
V
1 2
3.3V
V
OUT LR
V
.
LRFB
V
R
(
U
U
yields:
R
1
)
OUT(LR)
V
L
10k
R
V
L
LRFB
2
68pF
= 1.2 V and rearranging
k
1k
2.5V
LRFB1
LRDRV1
ADP3158/
ADP3178
(32)
(33)
Efficiency of the Linear Regulators
The efficiency and corresponding power dissipation of each
of the linear regulators are not determined by the controller
IC. Rather, these are a function of input and output voltage and
load current. Efficiency is approximated by the formula:
The corresponding power dissipation in the MOSFET, together
with any resistance added in series from input to output, is given by:
Minimum power dissipation and maximum efficiency are accom-
plished by choosing the lowest available input voltage that exceeds
the desired output voltage. However, if the chosen input source
is itself generated by a linear regulator, its power dissipation will
be increased in proportion to the additional current it must
now provide.
Implementing Current Limit for the Linear Regulators
The circuit of Figure 4 gives an example of a current limit pro-
tection circuit that can be used in conjunction with the linear
regulators. The output voltage is internally set by the LRFB
pin. The value of the current sense resistor may be calculated
as follows:
The power rating of the current sense resistor must be at least:
The maximum linear regulator MOSFET junction temperature
with a shorted output is:
which is within the maximum allowed by the MOSFET’s data
sheet specification. The maximum MOSFET junction tempera-
ture at nominal output is:
This example assumes an infinite heatsink. The practical limita-
tion will be based on the actual heatsink used.
T
T
T
T
J
J
J
J
(
(
(
(
P
R
P
MAX
MAX
NOM
NOM
LDO
D R S
S
(
100%
)
)
)
)
)
540
I
O MAX
T
T
50
50
(
V
(
A
A
R
IN
mV
C
C
S
(
(
V
–
V
)
OUT
J
V
J
( .
( .
I
C
IN
C
1 4
1 4
O MAX
OUT
(
540
V
(
V
C W
C W
2 2
IN
)
IN
.
/
/
)
mV
2
–
I
A
I
OUT
V
O MAX
OUT
( .
(
( .
1 2
3 3
3 3
.
)
250
W
V
V
)
)
I
– .
O NOM
m
(
2 5
2 2
.
V
A
)
)
)
)
2
60
A
C
)
52
C
(34)
(35)
(36)
(37)
(38)
(39)