response of the DC-DC converter. With ceramic capaci-

tors, the voltage ripple from ESL is negligible.

Output ripple is generated by variations in the charge

stored in the output capacitance, and the voltage drop

across the capacitor ESR.

The output voltage ripple due to the output capacitance is:

The output voltage ripple due to capacitor ESR is:

These equations are suitable for initial capacitor selec-

tion, but final values should be set by testing a proto-

type or evaluation circuit. As a rule, a smaller ripple

current results in less output voltage ripple. Because

the inductor ripple current is inversely proportional to

inductor value, output voltage ripple decreases with

larger inductance.

Load transient response depends on the selected output

capacitor. During a load transient, the output voltage

can respond, the output deviates further, depending on

the inductor and output capacitor values. After a short

time (see the Typical Operating Characteristics), the

controller responds by regulating the output voltage

back to its nominal state. The controller response time

depends on the closed-loop bandwidth. With a higher

bandwidth the response time is faster. However, to main-

tain stable operation, the bandwidth should not be set

Compensation Components

An internal transconductance error amplifier compen-

sates the control loop. Connect a series resistor and

capacitor between COMP and GND to form a pole-zero

pair. The external inductor, output capacitor, compen-

sation resistor, and compensation capacitor determine

the loop bandwidth and stability. The inductor and out-

put capacitor are chosen based on performance, size,

and cost. Additionally, the compensation resistor and

capacitor are selected to optimize the control loop.

Table 4 and Table 5 list typical component values. The

rest of this section is a more detailed discussion on cal-

culating compensation components.

The controller uses a current-mode control scheme that

regulates the output voltage by forcing the required

current through the external inductor. The voltage

across the internal high-side MOSFET’s on-resistance

is used to sense inductor current. Current-mode control

eliminates the double pole caused by the inductor and

output capacitor found in other control schemes.

stable and high-bandwidth loop.

then use the nearest standard value:

0.26

Ω (typ), and the transconductance from FB to

response performance, the closed-loop unity-gain fre-

tenth the switching frequency).

Below is a numerical example of calculating compen-

sation values for a circuit using the MAX1973 with 2.5V

output and maximum output current of 1A:

MAX1973

R

C

C

V

I

C

OUT

C

OUT

OUT MAX

=×

×

05

.

()

C

V

IR

g

f

C

FB

OUT MAX

CS

m

C

=

×

××

×

×

05

11

2

.

()

π

I

VV

fL

V

V

PP

IN

OUT

SW

OUT

IN

−

×

×

V

I

ESR

RIPPLE ESR

P P

−

V

I

Cf

RIPPLE C

PP

OUT

SW

××

−

8

VV

V

RIPPLE

RIPPLE C

RIPPLE ESR

=+

()

(

)

Smallest 1A, 1.4MHz Step-Down Regulators

______________________________________________________________________________________

13

MANUFACTURER

PART

INDUCTANCE (µH)

ESR (m

Ω)

SATURATION

CURRENT (A)

DIMENSIONS

L

Coilcraft

LPO1704-32M

3.3

160

1.3

5.5

Sumida

CDRD3D16-R3

3.3

85

1.1

4

Toko

A682AY-3R3M

3.3

134

0.97

4.4

Table 3. Recommended Inductors