NCP3101 ON Semiconductor, NCP3101 Datasheet - Page 9

• Manufacturer Part Number: NCP3101
• Description: Wide Input Voltage Synchronous Buck Converter
• Manufacturer: ON Semiconductor

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load. The controller immediately recognizes the load
transient and sets the duty cycle to maximum, but the current
slope is limited by the inductor value.
drops due to the current variation inside the capacitor and the
ESR. (neglecting the effect of the effective series inductance
(ESL)):
where V
effects of ESR and the ESR
resistance of the output capacitors.
of the NCP3101 demo board with the configuration shown
in Figure 19. The transient response was measured for the
load current step from 3 A to 6 A (50% to 100% load).
OS-CON, output capacitors are 2x100 mF ceramic and
OS-CON as mentioned in Table 1. Typical transient
response waveforms are shown in Figure 16.
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Table 1. TRANSIENT RESPONSE VERSUS OUTPUT
CAPACITANCE
Figure 16. Typical Waveform of Transient Response
C
During a load step transient the output voltage initially
Table 1 shows values of voltage drop and recovery time
Input capacitors are 2x47 mF ceramic and 1x270 mF
More information about OS-CON capacitors is available
OUT
(mF) OS-CON
OUT-ESR
1000
100
150
220
270
560
820
DV
0
OUT-ESR
is the voltage deviation of V
(50% to 100% Load Step)
+ DI
Drop (mV)
COUT
out
384
224
192
164
156
128
112
112
ESR
is the total effective series
COUT
Recovery Time (ms)
OUT
336
298
278
238
212
198
118
116
due to the
(eq. 6)
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NCP3101
9
current during the load transient without discharging it. The
voltage drop due to the output capacitor discharge is given
by the following equation:
DV
where V
due to the effects of discharge, L
value and V
Inductor Selection
the selection of an output inductor. From a mechanical
perspective, smaller inductor values generally correspond to
smaller physical size. Since the inductor is often one of the
largest components in the regulation system, a minimum
inductor
space-constrained
perspective, the maximum current slew rate through the
output inductor for a buck regulator is given by:
regulator's ability to slew current through the output
inductor in response to output load transients. Consequently,
output capacitors must supply the load current until the
inductor current reaches the output load current level. This
results in larger values of output capacitance to maintain
tight output voltage regulation. In contrast, smaller values of
inductance increase the regulator's maximum achievable
slew rate and decrease the necessary capacitance, at the
expense of higher ripple current. The peak-to-peak ripple
current is given by the following equation:
where Ipk-pk
From this equation it is clear that the ripple current increases
as L
dynamic response and ripple current. In order to achieve
high efficiency, coils with a low value of Direct Current
Resistance (DCR) have to be used.
Feedback and Compensation
shown in Table 1. The adjustment method requires an
external resistor divider with its center tap tied to the FB pin.
It is recommended to have a resistance between 1.5 kW and
5 kW. The selection of low value resistors reduces
efficiency, alternatively high value resistance of R2 causes
decrease in output voltage accuracy due to the bias current
in the error amplifier. The output voltage error of this bias
current can be estimated by using the following equation:
A minimum capacitor value is required to sustain the
Both mechanical and electrical considerations influence
This equation implies that larger inductor values limit the
The output voltage is adjustable from 0.8 V to 5 V as
OUT-DISCHARGE
OUT
OUT-DISCHARGE
decreases, emphasizing the trade-off between
Ipk-pk
IN
value
SlewRate
LOUT
is the input voltage.
LOUT
+
is the peak to peak current of the output.
applications.
2
is
LOUT
+
is the voltage deviation of V
C
L
particularly
+
OUT
OUT
V
DI
OUT
V
OUT
IN
OUT
( 1 * D )
L
* V
275 kHz
OUT
V
2
From
IN
is the output inductor
OUT
L
OUT
D * V
important
an
OUT
electrical
(eq. 7)
(eq. 8)
(eq. 9)
OUT
in