Datasheet

ADP3050 Data Sheet
Rev. C | Page 18 of 20
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.
00125-029
U1
ADP3050-5
V
IN
D1
1N5818
GND
12V
C1
22µF
C3
0.22µF
+
C2
0.01µF
R1
5.1kΩ
C4
3.3nF
D2
1N4148
+
C5
100µF
SD
–5V AT 0.5A
V
OUT
L1
47µH
1
2
3
4
8
7
6
5
SWITCH
BOOST
BIAS
FB
IN
GND
SD
COMP
Figure 29. Inverting (Buck-Boost) Regulator
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:
( )








+×××
×
−
×
+
=
OUT
IN
SW
OUT
IN
MAXSW
OUT
IN
IN
MAXOUT
VVLf
VV
I
VV
V
I
2
)(
)(
(18)
where I
SW(MAX)
is the switch current limit rating of the ADP3050,
and V
IN
is the minimum input voltage. The inductor ripple
current is estimated using the following equation:
OUT
MAXIN
OUT
SW
MAXIN
RIPPLE
VV
V
fL
V
I
+
××=
)(
)(
1
(19)
For the circuit in Figure 29, the maximum ripple current (at the
maximum input voltage) is
A375.0
512
5
10200
1
1047
12
36
=
−+
−
×
×
×
×
=
−
RIPPLE
I
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
IN
OUT
OUT
CCRMS
V
V
II
OUT
IN
×≈
),(
(20)
The peak current seen by the diode, switch, and inductor is
found by rearranging the load current equation






×+








×
+
=
RIPPLE
OUT
IN
OUT
IN
PEAK
II
V
VV
I
2
1
(21)
The largest peak currents occur at the lowest input voltage. For
this design with a load current of 500 mA
A9.0375.0
2
1
5.0
12
512
=






×+








×
−+
=
PEAK
I
(22)
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
IN
= 4 V or V
OUT
= −5 V, but the
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
IN
+ 2 × |V
OUT
|). It is important to ensure that the
maximum voltage rating of these pins is not exceeded.