ADP3170JRU Analog Devices, ADP3170JRU Datasheet - Page 13

• Manufacturer Part Number: ADP3170JRU
• Description: VRM 8.5 Compatible Single Phase Core Controller
• Manufacturer: Analog Devices

C
In continuous inductor-current mode, the source current of the
high-side MOSFET is a square wave with a duty ratio of V
V
current. To prevent large voltage transients, a low ESR input
capacitor sized for the maximum rms current must be used.
The maximum rms capacitor current is given by:
For a ZA-type capacitor with 1000 F capacitance and 6.3 V
voltage rating, the ESR is 24 m and the maximum allowable
ripple current at 100 kHz is 2 A. At 105 C, at least six such
capacitors must be connected in parallel to handle the calcu-
lated ripple current. At 50 C ambient, however, a higher
ripple current can be tolerated, so five capacitors in parallel
are adequate.
The ripple voltage across the five paralleled capacitors is:
To further reduce the effect of the ripple voltage on the system
supply voltage bus and to reduce the input-current di/dt to
below the recommended maximum of 0.1 A/ms, an additional
small inductor (L > 1 H @ 10 A) should be inserted between
the converter and the supply bus.
Linear Regulators
The linear regulator provides a low cost, convenient and versa-
tile solution for generating a 1.8 V supply rail. 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 and controlled by the ADP3170.
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 ADP3170 to provide a well regulated output voltage.
Output voltages higher than the fixed internal reference voltage
can be programmed by adding an external resistor divider.
Efficiency of the Linear Regulators
The efficiency and corresponding power dissipation of each
of the linear regulators are not determined by the ADP3170.
Rather, these are a function of input and output voltage and
load current. Efficiency is approximated by the formula:
lN
IN
Selection and Input Current di/dt Reduction
and an amplitude of one-half of the maximum output
V
V
C RIPPLE
C RIPPLE
(
(
I
I
C RMS
C RMS
)
)
(
(
23
I
O
)
)
A
ESR
23
I
n
O
24
C
A
C
5
m
D
100%
HSF
n
C
0 4 0 4
D
5 1
. – .
–
HSF MAX
C
D
IN
V
(
mF
V
HSF
OUT
IN
0 4
f
2
2
.
MIN
)
183
11 3
kHz
.
A
120
mV
OUT
(27)
(29)
(28)
/
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
accomplished 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
regulator. 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 heat sink. The practical limita-
tion will be based on the actual heat sink used.
T
T
T
T
1.8V, 2.2A
J
J
J
J
(
(
(
(
MAX
MAX
NOM
NOM
V
LR
)
)
)
)
T
50
50
50
A
100 F
R
P
o
250m
o
o
C
D R
C
C
S
1 F
R
(
P
S
LDO
S
J
)
540
I
C
1 4
1 4
O MAX
.
.
3.3V
J
(
C
R
o
V
o
m V
C W
S
C W
IN
V
/
/
V
IN
)
I
IN
O MAX
–
I
(
540
O MAX
V
–
10k
2 2
(
3 3
OUT
V
3 3
.
.
m V
OUT
.
)
2
A
V
V
)
68pF
1 2
– .
I
2 2
.
1k
OUT
1 8
.
250
I
W
O NOM
A
(
V
ADP3170
m
LRDRV
LRFB
60
1.8V
2
)
ADP3170
o
A
C
54
(30)
(31)
(32)
(33)
(34)
o
C