CMOS Micropower Step-Up
Switching Regulator
MAX630/MAX4193
Filter Capacitor
The output-voltage ripple has two components, with
approximately 90 degrees phase difference between
them. One component is created by the change in the
capacitor’s stored charge with each output pulse. The
other ripple component is the product of the capacitor’s
charge/discharge current and its effective series resis-
tance (ESR). With low-cost aluminum electrolytic
capacitors, the ESR-produced ripple is generally larger
than that caused by the change in charge.
⎛
V
⎞
V
ESR
=
I
PK
x ESR
= ⎜
IN
⎟
xESR(Voltsp
−
p)
⎝
2Lf
⎠
where V
IN
is the coil input voltage, L is its inductance, f
is the oscillator frequency, and ESR is the equivalent
series resistance of the filter capacitor.
The output ripple resulting from the change in charge
on the filter capacitor is:
Q
I
where, Q
=
t
DIS
x
PEAK
2
C
V
and, I
PEAK
=
t
CHG
x
IN
L
)(t )
V (t
V
dQ
=
IN CHG DIS
2LC
V
dQ
=
where t
CHG
and t
DIS
are the charge and discharge
times for the inductor (1/2f can be used for nominal cal-
culations).
When large values (>50kΩ) are used for the voltage-
setting resistors, R1 and R2 of Figure 1, stray capaci-
tance at the V
FB
input can add a lag to the feedback
response, destabilizing the regulator, increasing low-
frequency ripple, and lowering efficiency. This can
often be avoided by minimizing the stray capacitance
at the V
FB
node. It can also be remedied by adding a
lead compensation capacitor of 100pF to 10nF in paral-
lel with R1 in Figure 1.
DC-DC Converter Configurations
DC-DC converters come in three basic topologies:
buck, boost, and buck-boost (Figure 2). The MAX630 is
usually operated in the positive-voltage boost circuit,
where the output voltage is greater than the input.
The boost circuit is used where the input voltage is
always less than the desired output and the buck circuit
is used where the input is greater than the output. The
buck-boost circuit inverts, and can be used with, input
BOOST CONVERTER
+
V
BATT
S
1
CONTROL
SECTION
V
OUT
> V
BATT
-
Oscillator Capacitor, C
X
The oscillator capacitor, C
X
, is a noncritical ceramic or
silver mica capacitor. C
X
can also be calculated by:
C
X
=
2.14 X10
−
6
−
C
INT
(C
INT
≅
5pF, see text)
f
S
1
BUCK CONVERTER
+
V
BATT
CONTROL
SECTION
V
OUT
< V
BATT
where f is the desired operating frequency in Hertz, and
C
INT
is the sum of the stray capacitance on the C
X
pin
and the internal capacitance of the package. The internal
capacitance is typically 1pF for the plastic package and
3pF for the CERDIP package. Typical stray capacitances
are about 3pF for normal PC board layouts, but will be
significantly higher if a socket is used.
Bypassing and Compensation
Since the inductor-charging current can be relatively
large, high currents can flow through the ground con-
nection of the MAX630/MAX4193. To prevent unwanted
feedback, the impedance of the ground path must be
as low as possible, and supply bypassing should be
used for the device.
8
-
BUCK-BOOST CONVERTER
S
1
-
V
BATT
CONTROL
SECTION
|V
OUT
| < OR > V
BATT
+
Figure 2. DC-DC Converter Configurations
_______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
