ADP3212 ON Semiconductor, ADP3212 Datasheet - Page 33

• Manufacturer Part Number: ADP3212
• Description: 7-bit Programmable, 3-phase,mobile Cpu Synchronous Buck Controller
• Manufacturer: ON Semiconductor

RAMP RESISTOR SELECTION
The ramp resistor (R
ramp. The value of this resistor is chosen to provide the best
combination of thermal balance, stability, and transient response.
Use the following expression to determine a starting value:
where:
A
A
R
C
Another consideration in the selection of R
internal ramp voltage (see Equation 19). For stability and noise
immunity, keep the ramp size larger than 0.5 V. Taking this into
consideration, the value of R
The internal ramp voltage magnitude can be calculated as follows:
The size of the internal ramp can be increased or decreased. If it
is increased, stability and transient response improves but
thermal balance degrades. Conversely, if the ramp size is
decreased, thermal balance improves but stability and transient
response degrade. In the denominator of Equation 18, the factor
of 3 sets the minimum ramp size that produces an optimal
combination of good stability, transient response, and thermal
balance.
CURRENT LIMIT SETPOINT
To select the current limit setpoint, the resistor value for R
be determined. The current limit threshold for the
ADP3212/NCP3218 is set with R
the following equation:
DS
R
D
R
is the internal ramp amplifier gain.
is the internal ramp capacitor value.
is the current balancing amplifier gain.
is the total low-side MOSFET on resistance.
V
V
R
R
R
R
R
R
R
LIM
=
=
=
=
=
0
3
3
A
462
5 .
R
×
×
I
R
LIM
×
5
×
R
A
60
0
k
×
1 (
×
D
1 (
5 .
A
Ω
×
μ
5
×
C
−
R
−
R
×
A
2 .
) is used to set the size of the internal PWM
×
R
R
R
×
D
. 0
360
O
5
m
×
DS
L
)
061
pF
Ω
×
f
×
nH
SW
V
R
×
C
×
)
VID
in this example is selected as 280 kΩ.
×
5
R
280
. 1
pF
150
CLIM
kHz
=
. R
462
V
CLIM
=
k
. 0
R
Ω
can be found using
is the size of the
83
V
CLIM
must
Rev. SpA | Page 33 of 43
(18)
(19)
(20)
where:
R
R
I
When the ADP3212/NCP3218 is configured for 3 phase
operation, the equation above is used to set the current limit.
When the ADP3212/NCP3218 switches from 3 phase to 1 phase
operation by PSI or DPRSLP signal, the current is single phase
is one third of the current limit in 3 phase.
When the ADP3212/NCP3218 is configured for 2 phase
operation, the equation above is used to set the current limit.
When the ADP3212/NCP3218 switches from 2 phase to 1 phase
operation by PSI or DPRSLP signal, the current is single phase
is one half of the current limit in 2 phase.
When the ADP3212/NCP3218 is configured for 1 phase
operation, the equation above is used to set the current limit.
CURRENT MONITOR
The ADP3212/NCP3218 has output current monitor. The
IMON pin sources a current proportional to the total inductor
current. A resistor, R
of the output current monitor. A 0.1 μF is placed in parallel with
R
load transients. Since the IMON pin is connected directly to the
CPU, it is clamped to prevent it from going above 1.15V.
The IMON pin current is equal to the R
4. R
where:
R
IMON pin to FBRTN.
R
R
I
scale.
FEEDBACK LOOP COMPENSATION DESIGN
Optimized compensation of the ADP3212/NCP3218 allows the
best possible response of the regulator’s output to a load change.
The basis for determining the optimum compensation is to
make the regulator and output decoupling appear as an output
impedance that is entirely resistive over the widest possible
frequency range, including dc, and that is equal to the droop
resistance (R
voltage droops in proportion with the load current at any load
LIM
FS
LIM
O
MON
MON
LIM
O
is the output current when the voltage on IMON is at full
is the output load line.
is the output load line resistance.
MON
is the current limit setpoint.
is the current limit resistor.
is the current limit resistor.
R
to filter the inductor current ripple and high frequency
is the current monitor resistor. R
MON
can be found using the following equation:
=
O
. 1
). With the resistive output impedance, the output
4
15
×
V
R
O
×
MON
×
R
ADP3212/NCP3218
I
LIM
, from IMON to FBRTN sets the gain
FS
MON
LIM
times a fixed gain of
is connected from
(28)