MAX1966/MAX1967
Low-Cost Voltage-Mode PWM
Step-Down Controllers
10 ______________________________________________________________________________________
compromise between efficiency and economy. Choose
a low-loss inductor having the lowest possible DC
resistance. Ferrite-core-type inductors are often the
best choice for performance, however; the MAX1966/
MAX1967s’ 100kHz switching rate also allows the use
of powdered-iron cores in ultra-low-cost applications
where efficiency is not critical. With any core material,
the core must be large enough not to saturate at the
peak inductor current (I
PEAK
):
Setting the Current Limit
The MAX1966/MAX1967 provide current limit by sens-
ing the voltage across the external low-side MOSFET.
The current-limit threshold voltage is nominally -305mV.
The MOSFET on-resistance required to allow a given
peak inductor current is:
or
in terms of actual output current.
A limitation of sensing current across MOSFET resis-
tance is that current-limit threshold is not accurate
since the MOSFET R
DS(ON)
specification is not precise.
This type of current limit provides a coarse level of fault
protection. It is especially suited when the input source
is already current limited or otherwise protected.
However, since current-limit tolerance may be ±45%,
this method may not be suitable in applications where
this device’s current limit is the primary safety mecha-
nism, or where accurate current limit is required.
Output Capacitor Selection
The output filter capacitor must have low enough equiv-
alent series resistance (ESR) to meet output ripple and
load transient requirements, yet have high enough ESR
to satisfy stability requirements. In addition, the capaci-
tance value must be high enough to absorb the induc-
tor energy going from a full-load to no-load condition if
such load changes are anticipated in the system.
In applications where the output is subject to large load
transients, the output capacitor’s size depends primari-
ly on how low an ESR is needed to prevent the output
from dipping too low under load transients. Ignoring the
sag due to finite capacitance:
In applications with less severe load steps, the output
capacitor’s size may then primarily depend on how low
an ESR is required to maintain acceptable output ripple:
The actual capacitance value required relates to the
physical size and technology needed to achieve low
ESR. Thus, the capacitor is usually selected by physi-
cal size, ESR, and voltage rating rather than by capaci-
tance value. With current capacitor technology, once
the ESR requirement is satisfied, the capacitance is
usually also sufficient. When using a low-capacity filter
capacitor such as ceramic or polymer types, capacitor
size is usually determined by the capacitance needed
to prevent undershoot and overshoot voltages during
load transients. The overshoot voltage is given by:
Generally, once enough capacitance is added to meet
the overshoot requirement, undershoot at the rising
load edge is no longer a problem.
Stability and Compensation
To ensure stable operation, use the following compen-
sation procedure:
1) Determine accaptable output ripple and select the
inductor and output capacitor values as outlined in
the Inductor Selection and Output Capacitor
Selection sections.
2) Check to make sure that output capacitor ESR zero
is less than f
OSC
/π. Otherwise, increase capaci-
tance until this condition is satisfied.
3) Select R3 value to set high-frequency error-amplifi-
er gain so that the unity-gain frequency of the loop
occurs at the output ESR zero:
A good choice for R
3
is 50kΩ. Do not exceed 100kΩ.
