Datasheet

The input ripple comprises mainly of ΔV
Q
(caused by the
capacitor discharge) and ΔV
ESR
(caused by the ESR of
the input capacitor). The total voltage ripple is the sum of
ΔV
Q
and ΔV
ESR
. Assume the input voltage ripple from
the ESR and the capacitor discharge is equal to 50%
each. The following equations show the ESR and capaci-
tor requirement for a target voltage ripple at the input:
where
where I
OUT
is the output current, D is the duty cycle,
and f
SW
is the switching frequency. Use additional
input capacitance at lower input voltages to avoid pos-
sible undershoot below the UVLO threshold during
transient loading.
Output Capacitor Selection
The allowable output voltage ripple and the maximum
deviation of the output voltage during step load cur-
rents determine the output capacitance and its ESR.
The output ripple comprises of ΔV
Q
(caused by the
capacitor discharge) and ΔV
ESR
(caused by the ESR of
the output capacitor). Use low-ESR ceramic or alu-
minum electrolytic capacitors at the output. For alu-
minum electrolytic capacitors, the entire output ripple is
contributed by ΔV
ESR
. Use the ESR
OUT
equation to cal-
culate the ESR requirement and choose the capacitor
accordingly. If using ceramic capacitors, assume the
contribution to the output ripple voltage from the ESR
and the capacitor discharge to be equal. The following
equations show the output capacitance and ESR
requirement for a specified output voltage ripple.
where:
ΔI
P-P
is the peak-to-peak inductor current as calculated
above and f
SW
is the individual converter’s switching
frequency.
The allowable deviation of the output voltage during
fast transient loads also determines the output capaci-
tance and its ESR. The output capacitor supplies the
step load current until the controller responds with a
greater duty cycle. The response time (t
RESPONSE
)
depends on the closed-loop bandwidth of the convert-
er. The high switching frequency of the MAX15036/
MAX15037 allows for a higher closed-loop bandwidth,
thus reducing t
RESPONSE
and the output capacitance
requirement. The resistive drop across the output
capacitor’s ESR and the capacitor discharge causes a
voltage droop during a step load. Use a combination of
low-ESR tantalum and ceramic capacitors for better
transient load and ripple/noise performance. Keep the
maximum output voltage deviation below the tolerable
limits of the electronics being powered. When using a
ceramic capacitor, assume an 80% and 20% contribu-
tion from the output capacitance discharge and the
ESR drop, respectively. Use the following equations to
calculate the required ESR and capacitance value:
where I
STEP
is the load step and t
RESPONSE
is the
response time of the controller. The controller response
time depends on the control-loop bandwidth.
Boost Converter
The MAX15036 can be configured for step-up conver-
sion since the internal MOSFET can be used as a low-
side switch. Use the following equations to calculate
the inductor (L
MIN
), input capacitor (C
IN
), and output
capacitor (C
OUT
) when using the converter in boost
operation.
ESR
V
I
C
It
V
OUT
ESR
STEP
OUT
STEP RESPONSE
Q
=
=
×
Δ
Δ
Δ
ΔΔ
I
VV V
Vf L
VVV
PP
IN OUT OUT
IN SW
OUT RIPPLE ESR Q
−
=
−
()
×
××
≅+
_
ESR
V
I
C
I
8Vf
ESR
P-P
OUT
P-P
QSW
=
=
××
Δ
Δ
Δ
Δ
ΔI
VV V
Vf L
and
D
V
V
PP
IN OUT OUT
IN SW
OUT
IN
−
=
−
()
×
××
=
ESR
V
I
I
C
IDD
Vf
ESR
OUT
PP
IN
OUT
QSW
=
+
⎛
⎝
⎜
⎞
⎠
⎟
=
×−
()
×
−
Δ
Δ
Δ
2
1
MAX15036/MAX15037
2.2MHz, 3A Buck or Boost Converters
with an Integrated High-Side Switch
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