MAX1954A
Low-Cost, Current-Mode PWM Buck
Controller with Foldback Current Limit
14 ______________________________________________________________________________________
where V
IN
is the input voltage and f
S
is the switching
frequency. Choose a R
SNUB
power rating that meets
the specific application’s derating rule for the power
dissipation calculated.
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor must meet the ripple current
requirement (I
RMS
) imposed by the switching currents
defined by the following equation:
I
RMS
has a maximum value when the input voltage
equals twice the output voltage (V
IN
= 2 x V
OUT
); there-
fore, I
RMS(MAX)
= I
LOAD
/ 2. Ceramic capacitors are
recommended due to their low equivalent series resis-
tance (ESR) and equivalent series inductance (ESL) at
high frequencies, and their relatively low cost. Choose
a capacitor that exhibits less than 10°C temperature
rise at the maximum operating root-mean-square (RMS)
current for optimum long-term reliability.
Output Capacitor
The key selection parameters for the output capacitor
are the actual capacitance value, ESR, ESL, and the
voltage-rating requirements. These parameters affect
the overall stability, output voltage ripple, and transient
response. The output ripple has three components: vari-
ations in the charge stored in the output capacitor, and
the voltage drop across the capacitor’s ESR and ESL
caused by the current into and out of the capacitor. The
equation below estimates the maximum ripple voltage:
The output voltage ripple as a consequence of the ESR,
output capacitance, and ESL are as follows:
where I
P-P
is the peak-to-peak inductor current (see the
Inductor Value section). These equations are suitable
for initial capacitor selection, but final values should be
chosen based on a prototype or evaluation circuit. As a
general rule, a smaller current ripple results in less out-
put voltage ripple. Since the inductor ripple current is a
factor of the inductor value and input voltage, the out-
put voltage ripple decreases with larger inductance,
and increases with higher input voltages. For the
MAX1954A polymer, tantalum, or aluminum electrolytic
capacitors are recommended. Lower-cost aluminum
electrolytic capacitors with relatively low ESR are avail-
able and can be used for the MAX1954A, if the larger
physical size is acceptable. For reliable and safe oper-
ation, ensure that the capacitor’s voltage and ripple-
current ratings exceed the calculated values.
The devices’ response to a load transient depends on
the selected output capacitors. After a load transient,
the output voltage instantly changes by ESR x ∆I
LOAD
.
Before the controller can respond, the output voltage
deviates further depending on the inductor and output
capacitor values. After a short period of 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
its closed-loop bandwidth. With a higher bandwidth, the
response time is faster, thus preventing the output volt-
age from deviating further from its regulation value.
Compensation Design
The MAX1954A uses an internal transconductance
error amplifier whose output compensates the control
loop. The external inductor, high-side MOSFET, output
capacitor, compensation resistor, and compensation
capacitors determine the loop stability. The inductor
and output capacitors are chosen based on perfor-
mance, size, and cost. Additionally, the compensation
resistor and capacitors are selected to optimize control-
loop stability. The component values in Figures 1 and 2
yield stable operation over the given range of input-to-
output voltages and load currents. The controller uses a
current-mode control scheme that regulates the output
voltage by forcing the required current through the
external inductor. The MAX1954A uses the voltage
across the high-side MOSFET’s on-resistance
(R
DS(ON)
) to sense the inductor current. Current-mode
control eliminates the double pole in the feedback loop
caused by the inductor and output capacitor, resulting
in a smaller phase shift and requiring less elaborate
error-amplifier compensation. A single-series compen-
sation resistor (R
C
) and compensation capacitor (C
C
) is
all that is needed to have a stable high-bandwidth loop
in applications where ceramic capacitors are used for
