Datasheet

MAX16936
36V, 220kHz to 2.2MHz Step-Down Converter
with 28µA Quiescent Current
13Maxim Integrated
with high ripple current capability at the input. Assume
the contribution from the ESR and capacitor discharge
equal to 50%. Calculate the input capacitance and ESR
required for a specified input voltage ripple using the fol-
lowing equations:
ESR
IN
L
OUT
V
ESR
I
I
2
∆
=
∆
+
where:
SUP OUT OUT
L
SUP SW
(V V ) V
I
V fL
−×
∆=
××
and:
OUT OUT
IN
Q SW SUPSW
I D(1 D) V
C and D
Vf V
×−
= =
∆×
where I
OUT
is the maximum output current and D is the
duty cycle.
Output Capacitor
The output filter capacitor must have low enough ESR
to meet output ripple and load transient requirements.
The output capacitance must be high enough to absorb
the inductor energy while transitioning from full-load
to no-load conditions without tripping the overvoltage
fault protection. When using high-capacitance, low-ESR
capacitors, the filter capacitor’s ESR dominates the output
voltage ripple. So the size of the output capacitor depends
on the maximum ESR required to meet the output-voltage
ripple (V
RIPPLE(P-P)
) specifications:
RIPPLE(P P) LOAD(MAX)
V ESR I LIR
−
=××
The actual capacitance value required relates to the
physical size needed to achieve low ESR, as well as
to the chemistry of the capacitor technology. Thus, the
capacitor is usually selected by ESR and voltage rating
rather than by capacitance value.
When using low-capacity filter capacitors, such as ceramic
capacitors, size is usually determined by the capacity
needed to prevent voltage droop and voltage rise from
causing problems during load transients. Generally,
once enough capacitance is added to meet the over-
shoot requirement, undershoot at the rising load edge
is no longer a problem. However, low capacity filter
capacitors typically have high ESR zeros that can affect
the overall stability.
Rectifier Selection
The device requires an external Schottky diode rectifier
as a freewheeling diode when the device is configured
for skip mode operation. Connect this rectifier close to the
device using short leads and short PCB traces. In FPWM
mode, the Schottky diode helps minimize efficiency loss-
es by diverting the inductor current that would otherwise
flow through the low-side MOSFET. Choose a rectifier
with a voltage rating greater than the maximum expected
input voltage, V
SUPSW
. Use a low forward-voltage-drop
Schottky rectifier to limit the negative voltage at LX. Avoid
higher than necessary reverse-voltage Schottky rectifiers
that have higher forward-voltage drops.
Compensation Network
The device uses an internal transconductance error ampli-
fier with its inverting input and its output available to the
user for external frequency compensation. The output
capacitor and compensation network determine the loop
stability. The inductor and the output capacitor are chosen
based on performance, size, and cost. Additionally, the
compensation network optimizes the control-loop stability.
The controller uses a current-mode control scheme that
regulates the output voltage by forcing the required cur-
rent through the external inductor. The device uses the
voltage drop across the high-side MOSFET to sense
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
than voltage-mode control. Only a simple single-series
resistor (R
C
) and capacitor (C
C
) are required to have a
stable, high-bandwidth loop in applications where ceramic
capacitors are used for output filtering (Figure 4). For other
types of capacitors, due to the higher capacitance and
ESR, the frequency of the zero created by the capacitance
Figure 4. Compensation Network
R2
R1
V
REF
V
OUT
R
C
C
C
C
F
COMP
g
m