ADP3050 Analog Devices, ADP3050 Datasheet - Page 15

• Manufacturer Part Number: ADP3050
• Description: 200 kHz, 1 A Step-Down High-Voltage Switching Regulator
• Manufacturer: Analog Devices

Ceramics are an excellent choice for input bypassing, due to
their low ESR and high ripple current rating. Ceramics are
especially suited for high input voltages and are available from
many different manufacturers. Tantalums are often used for
input bypassing, but precautions must be taken because they
occasionally fail when subjected to large inrush currents during
power-up. These surges are common when the regulator input
is connected to a battery or high capacitance supply. Several
manufacturers now offer surface-mount solid tantalum capacitors
that are surge tested, but even these devices can fail if the current
surge occurs when the capacitor voltage is near its maximum
rating. For this reason, a 2:1 derating is suggested for tantalum
capacitors used in applications where large inrush currents are
present. For example, a 20 V tantalum should be used only for
an input voltage up to 10 V. Aluminum electrolytics are the
cheapest choice, but it takes several in parallel to get a good rms
current rating. OS-CON capacitors have a good ESR and ripple
current rating, but they are typically larger and more costly.
Refer to Table 4 for a list of capacitor manufacturers.
DISCONTINOUS MODE RINGING
When operating in discontinuous mode, high frequency
ringing appears at the switch node when the inductor current
has decreased to zero. This ringing is normal and is not a result
of loop instability. It is caused by the switch and diode capacitance
reacting with the inductor to form a damped sinusoidal ringing.
This ringing is usually in the range of several megahertz, and is
not harmful to normal circuit operation.
SETTING THE OUTPUT VOLTAGE
The fixed voltage versions of the ADP3050 (3.3 V and 5 V) have
the feedback resistor divider included on-chip. For the adjustable
version, the output voltage is set using two external resistors.
Referring to Figure 25, pick a value for R1 between 10 kΩ and
20 kΩ, then calculate the appropriate value for R2 using the
following equation:
It is important to note that the accuracy of these resistors
directly affects the accuracy of the output voltage. The FB pin
threshold variation is ±3%, and the tolerances of R1 and R2 add
to this to determine the total output variation. Use 1% resistors
placed close to the FB pin to prevent noise pickup.
FREQUENCY COMPENSATION
The ADP3050 uses a unique compensation scheme that allows
the use of any type of output capacitor. The designer is not
limited to a specific type of capacitor or a specific ESR range.
External compensation allows the designer to optimize the loop
for transient response and system performance. The values for
R
to compensate the regulator loop.
C
and C
R2
=
C
R1
set the pole and zero locations for the error amplifier
×
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⎜
⎝
V
. 1
OUT
20
−
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Rev. B | Page 15 of 24
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For tantalum output capacitors, the typical system compensation
values are R
values are R
optimized for all designs, but they provide a good starting point for
selecting the final compensation values. Other types of output
capacitors require different values of C
Typically, the lower the ESR of the output capacitor, the larger
the value for C
capacitance, and inductor value (due to production tolerances,
changes in operating point, changes in temperature) affect the
loop gain and phase response. Always check the final design
over its complete operating range to ensure proper regulator
operation.
Adjusting the R
the typical values above as a starting point, then try increasing
and decreasing each independently and observing the transient
response. An easy way to check the transient response of the
design is to observe the output while pulsing the load current at
a rate of approximately 100 Hz to 1 kHz. There should be some
slight ringing at the output when the load pulses, but this should
not be excessive (just a few rings). The frequency of this ringing
shows the approximate unity-gain frequency of the loop. Again,
always check the design over its full operating range of input
voltage, output current, and temperature to ensure that the loop
is compensated correctly.
In addition to setting the zero location, R
frequency gain of the error amplifier. If this gain is too large,
output ripple voltage appears at the COMP pin (the output of
the error amplifier) with enough amplitude to interfere with
normal regulator operation. If this occurs, subharmonic switching
results (the pulsewidth of the switch waveform changes, even
though the output voltage stays regulated). The voltage ripple at the
COMP pin should be kept below 100 mV to prevent subharmonic
switching from occurring. The amount of ripple can be estimated
by the following formula, where g
transconductance (g
For example, a 12 V to 5 V, 800 mA regulator with an inductor of
L = 47 μH has I
Mode section) if a 100 μF tantalum output capacitor with a
maximum ESR of 100 mΩ and compensation values of R
and C
If this ripple voltage is more than 100 mV, R
decreased to prevent subharmonic switching. Typical values for
R
C
are in the range of 2 kΩ to 10 kΩ.
V
V
C
COMP
COMP
= 1 nF are used. The ripple voltage at the COMP pin is
,
,
C
C
RIPPLE
RIPPLE
= 4 kΩ and C
= 4 kΩ and C
C
RIPPLE
C
. Normal variations in capacitor ESR, output
and C
=
=
=
(
(
37
= 310 mA (see example from the Continuous
1250
g
m
m
2 .
= 1250 μMho):
C
×
mV
values can optimize compensation. Use
R
×
10
C
C
C
) (
= 1 nF; for ceramics, the typical
= 4.7 nF. These values may not be
×
−
6
×
I
RIPPLE
4
m
×
is the error amplifier
10
C
between 0.5 nF and 10 nF.
×
3
) (
ESR
×
C
also sets the high
. 0
)
C
310
×
needs to be
V
V
OUT
×
FB
ADP3050
0
1 .
)
×
C
= 4 kΩ
. 1
5
20
0 .
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