Datasheet

An R
LIM
resistance range of 6kΩ to 60kΩ corresponds to
a current-limit threshold of 30mV to 300mV. Use 1% toler-
ance resistors when adjusting the current limit to minimize
error in the current-limit threshold.
Input Capacitor
The input filter capacitor reduces peak current drawn from
the power source and reduces noise and voltage ripple
on the input caused by the switching circuitry. The input
capacitor must meet the ripple current requirement (I
RMS
)
imposed by the switching currents as defined by the fol-
lowing equation:
OUT IN OUT
RMS LOAD(MAX)
IN
V (V - V )
II
V
=
I
RMS
attains a maximum value when the input volt-
age equals twice the output voltage (V
IN
= 2V
OUT
),
so I
RMS(MAX)
= I
LOAD(MAX)/2
. For most applications,
nontantalum capacitors (ceramic, aluminum, polymer, or
OS-CON) are preferred at the inputs due to the robust-
ness of nontantalum capacitors to accommodate high
inrush currents of systems being powered from very low
impedance sources. Additionally, two (or more) smaller-
value low-ESR capacitors should be connected in parallel
to reduce high-frequency noise.
Output Capacitor
The key selection parameters for the output capacitor
are capacitance value, ESR, and voltage rating. These
parameters affect the overall stability, output ripple volt-
age, and transient response. The output ripple has two
components: variations in the charge stored in the output
capacitor, and the voltage drop across the capacitor’s ESR
caused by the current flowing into and out of the capacitor:
ΔV
RIPPLE
= ΔV
ESR
+ ΔV
Q
The output-voltage ripple as a consequence of the ESR
and the output capacitance is:
ESR P-P
P-P
Q
OUT SW
IN OUT OUT
P-P
SW IN
V I ESR
I
V
8C f
V -V V
I
fL V
∆=×
∆=
××


= ×


×


where I
P-P
is the peak-to-peak inductor current ripple
(see the Inductor Selection section). Use these equations
for initial capacitor selection. Decide on the final values by
testing a prototype or an evaluation circuit.
Check the output capacitor against load-transient
response requirements. The allowable deviation of the
output voltage during fast load transients determines the
capacitor output capacitance, ESR, and equivalent series
inductance (ESL). The output capacitor supplies the load
current during a load step until the controller responds
with a higher duty cycle. The response time (t
RESPONSE
)
depends on the closed-loop bandwidth of the converter
(see the Compensation Design section). The resistive
drop across the ESR of the output capacitor, the voltage
drop across the ESL (ΔV
ESL
) of the capacitor, and the
capacitor discharge, cause a voltage droop during the
load step.
Use a combination of low-ESR tantalum/aluminum elec-
trolytic and ceramic capacitors for improved transient load
and voltage ripple performance. Nonleaded capacitors
and capacitors in parallel help reduce the ESL. Keep the
maximum output-voltage deviation below the tolerable
limits of the load. Use the following equations to calculate
the required ESR, ESL, and capacitance value during a
load step:
ESR
STEP
STEP RESPONSE
OUT
Q
ESL STEP
STEP
RESPONSE
O
V
ESR
I
It
C
V
Vt
ESL
I
1
t
3f
∆
=
×
=
∆
∆×
=
≅
×
where I
STEP
is the load step, t
STEP
is the rise time of the
load step, t
RESPONSE
is the response time of the control-
ler, and f
O
is the closed-loop crossover frequency.
Compensation Design
The MAX15046 provides an internal transconductance
amplifier with the inverting input and the output avail-
able for external frequency compensation. The flexibility
of external compensation offers wide selection of output
filtering components, especially the output capacitor.
Use high-ESR aluminum electrolytic capacitors for cost-
sensitive applications. Use low-ESR tantalum or ceramic
capacitors at the output for size-sensitive applications. The
high switching frequency of the MAX15046 allows the use
of ceramic capacitors at the output. Choose all passive
power components to meet the output ripple, component
size, and component cost requirements. Choose the com-
pensation components for the error amplifier to achieve
the desired closed-loop bandwidth and phase margin.
MAX15046 40V, High-Performance, Synchronous
Buck Controller
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