ADP3050 Analog Devices, ADP3050 Datasheet - Page 19

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

INVERTING (BUCK BOOST) REGULATOR
The circuit in Figure 29 shows the ADP3050 in a buck-boost
configuration that produces a negative output voltage from a
positive input voltage. This topology looks quite similar to the
buck shown in Figure 28 (except the IC and the output filter are
now referenced to the negative output instead of ground), but
its operation is quite different. For this topology, the feedback
pin is grounded and the GND pin is tied to the negative output,
allowing the feedback network of the IC to regulate the negative
output voltage.
The design procedure used for the standard buck converter
cannot be used for a buck-boost converter due to fundamental
differences in how the output voltage is generated. The switch
currents in the buck-boost are much higher than the standard
buck converter, thus lowering the available load current. To
calculate the maximum output current for a given maximum
switch current, use the following equation:
where I
and V
current is estimated using the following equation:
SD
V
1N5818
GND
IN
47µH
12V
I
⎡
⎢
⎢
⎣
I
IN
L1
OUT
I
RIPPLE
D1
SW
SW(MAX)
is the minimum input voltage. The inductor ripple
(
(
MAX
MAX
=
+
C3
0.22µF
V
1N4148
)
Figure 29. Inverting (Buck-Boost) Regulator
)
is the switch current limit rating of the ADP3050,
C1
22µF
=
−
IN
V
2
D2
(
L
MAX
×
IN
f
V
+
SW
)
IN
V
×
V
×
OUT
C2
0.01µF
f
1
2
3
4
IN
L
SW
1
ADP3050-5
SWITCH
BOOST
BIAS
FB
×
×
×
(
×
V
V
V
U1
OUT
IN
IN
COMP
+
GND
(
MAX
SD
V
IN
V
OUT
OUT
)
8
7
6
5
5.1kΩ
+
)
V
R1
⎤
⎥
⎥
⎦
OUT
C4
3.3nF
100µF
C5
–5V AT 0.5A
+
V
OUT
(18)
(19)
Rev. B | Page 19 of 24
For the circuit in Figure 29, the maximum ripple current (at the
maximum input voltage) is
High ripple currents are present in both the input and output
capacitors, and their ripple current ratings must be large
enough to sustain the large switching currents present in this
topology. The capacitors should have a ripple current rating of
at least
The peak current seen by the diode, switch, and inductor is
found by rearranging the load current equation
The largest peak currents occur at the lowest input voltage. For
this design with a load current of 500 mA
The average current diode is equal to the load current.
An inductor with a current rating 20% greater than 0.9 A
should be used (a rating of at least 1.2 A). Inductors and diodes
with ratings greater than 1 A should always be used, even if
the calculated peak and average currents are lower. This ensures
that start-up and fault conditions do not overstress the
components.
For the buck-boost topology, the input voltage can be less than
the output voltage, such as V
available load current is even lower. The equations given in this
section are valid for input voltages less than and greater than
the output voltage. The voltage seen by the ADP3050 is equal to
the sum of the input and output voltages (the BOOST pin sees
the sum of V
maximum voltage rating of these pins is not exceeded.
I
I
I
RMS
PEAK
PEAK
I
RIPPLE
(
C
=
=
IN
,
⎛
⎜
⎜
⎝
⎛
⎜ ⎜
⎝
IN
C
V
12
OUT
=
+ 2 × |V
IN
47
)
+
12
≈
V
+
−
×
12
I
IN
V
10
5
OUT
OUT
×
−
OUT
6
0
×
5 .
×
×
|). It is important to ensure that the
⎞
⎟ ⎟
⎠
200
I
V
+
IN
V
OUT
OUT
⎛ ×
⎜
⎝
= 4 V or V
IN
1
×
1
2
10
⎞
⎟
⎟
⎠
+
. 0
3
⎛ ×
⎜
⎝
×
375
1
2
12
I
⎞
⎟
⎠
OUT
+
−
RIPPLE
=
5
−
0
= −5 V, but the
9 .
5
A
=
⎞
⎟
⎠
. 0
ADP3050
375
A
(20)
(21)
(22)