ADP3208C ON Semiconductor, ADP3208C Datasheet - Page 36

• Manufacturer Part Number: ADP3208C
• Description: 7-bit,programmable,dual- Phase,mobile,cpu,synchronous Buck Controller
• Manufacturer: ON Semiconductor

ADP3208C
Current Limit Setpoint
To select the current-limit setpoint, we need to find the resistor
value for R
set when the current in R
current of 20 μA. The current in R
current times R
equation:
where:
R
ILIM pin to ground.
R
I
current that will trip current limit.
In this example, if choosing 55 A for I
which is close to a standard 1% resistance of 5.76 kΩ.
The per-phase current limit described earlier has its limit
determined by the following:
For the ADP3208C, the maximum COMP voltage (V
is 3.3 V, the COMP pin bias voltage (V
current balancing amplifier gain (A
and a R
results in a per-phase limit of 85 A. Although this number
seems high, this current level can only be reached with a
absolute short at the output and the current-limit latch-off
function shutting down the regulator before overheating occurs.
This limit can be adjusted by changing the ramp voltage V
However, users should not set the per-phase limit lower than
the average per-phase current (I
There is also a per-phase initial duty-cycle limit at maximum
input voltage:
For this example, the duty-cycle limit at maximum input
voltage is found to be 0.25 when D is 0.061.
Output Current monitor
The ADP3208C has output current monitor. The IMON pin
sources a current proportional to the total inductor current. A
resistor, R
output current monitor. A 0.1 μF is placed in parallel with R
LIM
LIM
O
is the output load line resistance.
is the current limit set point. This is the peak inductor
is the current limit resistor. R
D
R
I
PHLIM
LIM
LIM
DS(MAX)
MON
=
LIM
=
≅
D
V
I
. The current-limit threshold for the ADP3208C is
, from IMON to FBRTN sets the gain of the
MIN
of 3.8 mΩ (low-side on-resistance at 150°C)
LIM
COMP
O
20
. R
×
A
μ
×
(
V
D
MAX
LIM
A
COMP
×
R
R
O
)
can be found using the following
−
DS
(
V
MAX
(
LIM
MAX
V
R
R
−
)
is equal to the internal reference
)
V
−
BIAS
V
LIM
BIAS
LIM
+
LIM
/n).
D
I
2
) is 5. Using a V
R
is connected from the
is equal to the inductor
LIM
BIAS
, R
) is 1.0 V, and the
LIM
is 5.775 kΩ,
R
Rev. 1 | Page 36 of 41 | www.onsemi.com
COMP(MAX)
of 0.55 V,
R
.
MON
(25)
(26)
(27)
)
to filter the inductor current ripple and high frequency 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
10. R
where:
R
IMON pin to FBRTN.
R
R
I
scale.
Feedback Loop Compensation Design
Optimized compensation of the ADP3208C 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
current slew rate, ensuring the optimal position and allowing
the minimization of the output decoupling.
With the multimode feedback structure of the ADP3208C, it is
necessary to set the feedback compensation so that the
converter’s output impedance works in parallel with the output
decoupling. In addition, it is necessary to compensate for the
several poles and zeros created by the output inductor and
decoupling capacitors (output filter).
A Type III compensator on the voltage feedback is adequate
for proper compensation of the output filter. Figure 37 shows the
Type III amplifier used in the ADP3208C. Figure 38 shows the
locations of the two poles and two zeros created by this amplifier.
FS
MON
LIM
O
is the output current when the voltage on IMON is at full
is the output load line resistance.
is the current limit resistor.
MON
R
is the current monitor resistor. R
MON
can be found using the following equation:
=
O
10
. 1
). With the resistive output impedance, the output
15
×
V
R
O
×
×
R
I
LIM
FS
MON
LIM
times a fixed gain of
is connected from
(28)