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:
I
P-P
is the peak-to-peak inductor current:
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
instantly changes by ESR ✕ ∆I
LOAD
. Before the controller
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
above f
SW
/10.
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.
Simple Type 1 compensation with a single resistor (R
C
)
and capacitor (C
C
) is all that is needed to provide a
stable and high-bandwidth loop.
Use the formula below to calculate the value of C
C
,
then use the nearest standard value:
where V
FB
is 1.25V for the MAX1973 and 0.75V for the
MAX1974, the current-sense transresistance (R
CS
) is
0.26Ω (typ), and the transconductance from FB to
COMP (g
m
) is 50µS (typ). For best stability and
response performance, the closed-loop unity-gain fre-
quency (f
C
) should be approximately 140kHz (one-
tenth the switching frequency).
Use the following equation to calculate R
C
:
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
