ADP3170JRU Analog Devices, ADP3170JRU Datasheet - Page 8

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

ADP3170
CT Selection for Operating Frequency
The ADP3170 uses a constant off-time architecture with t
determined by an external timing capacitor CT. Each time the
high-side N-channel MOSFET switch turns on, the voltage
across CT is reset to approximately 0 V. During the off-time,
CT is charged by a constant current of 150 A. Once CT reaches
3.0 V, a new on-time cycle is initiated. The value of the off-time is
calculated using the continuous-mode operating frequency.
Assuming a nominal operating frequency (f
at an output voltage of 1.8 V, the corresponding off-time is:
The timing capacitor cab be calculated from the equation:
The converter operates at the nominal operating frequency only
at the above-specified V
V
due to the parasitic voltage drops across the power devices. The
actual minimum frequency at V
183 kHz (see Equation 3), where:
Inductance Selection
The choice of inductance determines the ripple current in the
inductor. Less inductance leads to more ripple current, which
increases the output ripple voltage and the conduction losses in
the MOSFETs, but allows using smaller-size inductors and, for
a specified peak-to-peak transient deviation, output capacitors
with less total capacitance. Conversely, a higher inductance
means lower ripple current and reduced conduction losses, but
requires larger-size inductors and more output capacitance for
the same peak-to-peak transient deviation. The following equa-
tion shows the relationship between the inductance, oscillator
frequency, peak-to-peak ripple current in an inductor and input
and output voltages:
OUT
R
(estimated value: 6 m )
R
(estimated value: 6 m )
R
(estimated value: 2.5 m )
R
(estimated value: 3 m )
SENSE
L
DS(ON)HSF
DS(ON)LSF
, or under heavy load, the operating frequency decreases
is the resistance of the inductor
C
is the resistance of the sense resistor
T
is the resistance of the high-side MOSFET
is the resistance of the low-side MOSFET
t
OFF
t
V
OFF
1
L
T TH
–
(
1 8
I
5
.
V
)
CT
OUT
V
I
OUT
1
V
L RIPPLE
–
(
and at light load. At higher values of
3 3
f
V
MIN
V
. 3 2
.
OUT
1
IN
t
200
OUT
OFF
s
)
s
1
3
t
kHz
= 1.8 V is calculated to be
OFF
V
5
1
150
f
V
NOM
1
–
5
V
23
3 2
A
V
.
IN
NOM
V
–
A
–
IN
150
23
s
) of 200 kHz
I
O MAX
–
(
(
A
6
I
pF
O MAX
m
(
(
)
6
OFF
m
2 5
)
(1)
(2)
(4)
R
.
DS ON HSF
m
R
(
3
DS ON HSF
m
(
)
VID3
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
For 6 A peak-to-peak ripple current, which corresponds to
approximately 25% of the 23 A full-load dc current in an inductor,
Equation 4 yields an inductance of:
A 1 H inductor can be used, which gives a calculated ripple
current of 5.9 A at no load. The inductor should not saturate at
the peak current of 26 A and should be able to handle the sum
of the power dissipation caused by the average current of 23 A
in the winding and the core loss.
3
) – .
)
m
1 8
R
–
SENSE
V
6
R
VID2
1
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
SENSE
m
Table I. Output Voltage vs. VID Code
) )
R
L
183 kHz
L
R
–
VID1
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
L
R
DS ON LSF
1 8
–
.
V
(
OUT
V
6
)
A
VID0
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
3 3
.
s
990
VID25
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
nH
V
1.050 V
1.075 V
1.100 V
1.125 V
1.150 V
1.175 V
1.200 V
1.225 V
1.250 V
1.275 V
1.300 V
1.325 V
1.350 V
1.375 V
1.400 V
1.425 V
1.450 V
1.475 V
1.500 V
1.525 V
1.550 V
1.575 V
1.600 V
1.625 V
1.650 V
1.675 V
1.700 V
1.725 V
1.750 V
1.775 V
1.800 V
1.825 V
OUT(NOM)
(3)