ADP3050-3.3 Analog Devices, ADP3050-3.3 Datasheet - Page 12

• Manufacturer Part Number: ADP3050-3.3
• Description: 200 Khz, 1 a Step-down High-voltage Switching Regulator
• Manufacturer: Analog Devices

ADP3050
short). This is usually only a problem when the input voltage to out-
put voltage ratio is greater than 2.5. Under this condition, the load
current needed is slightly more than the regulator can provide. The
output voltage then droops slightly, and the switch stays on every
cycle until the internal current limit is reached. Under this condition,
the load current can reach around 1.2 A. For example: a system with
an input voltage of 24 V and an output voltage of 5 V, if a gradual
overload causes the output voltage to droop to 4 V, the average
diode current would be:
If the system must survive such gradual overloads for a prolonged
period of time, ensure that the diode chosen can survive these
conditions. A larger 2 A or 3 A diode can be used if necessary.
Choosing a Catch Diode
Use the following steps to pick an appropriate catch diode.
Table V shows several Schottky rectifiers with different reverse
voltage and forward current ratings.
1. The average diode current rating must be sufficient to provide
2. The reverse voltage rating of the catch diode should be at
INPUT CAPACITOR SELECTION
The input bypass capacitor plays an important role in proper
regulator operation, minimizing voltage transients at the input
and providing a short local loop for the switching current. Place
this capacitor close to ADP3050 between the V
using short, wide traces. This input capacitor should have an rms
ripple current rating of at least:
V
15 V
20 V
30 V
40 V
R
the required load current (see above calculations). Diodes
rated below 1 A should not be used, even if the average diode
current is much lower.
least the maximum input voltage. Often a higher rating is
chosen (1.2 times the maximum input voltage) to provide a
safety margin.
Table V. Schottky Diode Selection Guide
1 A Diodes
10BQ15
1N5817
SK12
B120
EC10QS02
V1N5818
SK13
B130
MBRS130
EC10QS03
1N5819
SK14
B140
MBRS140
10BQ040
EC10QS04
I
CI
I
N(RMS)
DIODE(AVG)
≥
2 A Diodes
30BQ15
B220
B230
EC21QS03
NSQ03A03
B240
EC21QS04
I
OUT
=
1 2
×
.
V
×
V
OUT
24 4
IN
24
3 A Diodes
SK32
B320
NSQ03A02
SK33
B330
SK34
B340
MBRS340
30BQ040
NSQ03A04
–
−



V
V
=
OUT
IN
1 0
IN
.



and GND pins
A
2
Company
Motorola
Diodes, Inc.
International Rectifier
Nihon Inter Electronic (USA)
Nihon Inter Electronic (Japan)
MBRSxxx
1N58xx, Bxxx, SKxxx
10BQxxx, 30BQxxx
ECxxxxxx, NSQxxxx
(6)
This rating is crucial, as the input capacitor must be able to
withstand the large current pulses present at the input of a step-
down regulator. Values of 20 µF to 50 µF are typical, but the
main criteria for capacitor selection should be the ripple current
and voltage ratings.
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 differ-
ent manufacturers. Tantalums are often used for input bypassing,
but precautions must be taken, as 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 (AVS TPS series, Sprague 593D series, Kemet T495 series,
and NEMCO LSR series), but even these devices can fail if the
current surge occurs when the capacitor voltage is near its maxi-
mum rating. For this reason, a 2:1 derating is suggested for
tantalum capacitors used in applications where large inrush currents
are present (i.e., a 20 V tantalum should only be used 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 IV
for a list of capacitor manufacturers.
DISCONTINUOUS MODE RINGING
When operating in discontinuous mode, high frequency ringing
will appear 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 2, pick a value for R1 between 10 kΩ and
20 kΩ, then calculate the appropriate value for R2 using:
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 will add to this
Table VI. Schottky Diode Manufacturers
Phone
[phone]
[phone]
[phone]
[phone]
81-33-343-3411 81-33-343-7377
R
2
=
R
1
×



V
1 20
OUT
.
Fax
[phone] www.mot.com
[phone] www.diodes.com
[phone] www.irf.com
[phone]
−
1



(7)