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.

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:

times for the inductor (1/2f can be used for nominal cal-

culations).

where f is the desired operating frequency in Hertz, and

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.

When large values (>50k

Ω) are used for the voltage-

setting resistors, R1 and R2 of Figure 1, stray capaci-

response, destabilizing the regulator, increasing low-

frequency ripple, and lowering efficiency. This can

often be avoided by minimizing the stray capacitance

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

C

X

f

C

C

pF see text

X

INT

INT

=−

−

≅

214 10

5

6

.

(,

)

V

Q

C

where Q

t

x

I

and I

t

x

V

L

V

Vt

t

LC

dQ

DIS

PEAK

PEAK

CHG

IN

dQ

IN CHG

DIS

==

=

=

,

,

()(

)

2

2

V

I

x ESR

V

Lf

xESR Voltsp p

ESR

PK

IN

==⎛

⎝⎜

⎞

⎠⎟

−

2

()

CMOS Micropower Step-Up

Switching Regulator

8

_______________________________________________________________________________________

CONTROL

SECTION

+

-

BOOST CONVERTER

CONTROL

SECTION

+

-

BUCK CONVERTER

CONTROL

SECTION

+

-

BUCK-BOOST CONVERTER

Figure 2. DC-DC Converter Configurations