Max5099 – Rainbow Electronics MAX5099 User Manual
Page 19
MAX5099
Output Capacitor
The allowable output ripple voltage and the maximum
deviation of the output voltage during step load cur-
rents determine the output capacitance and its ESR.
The output ripple is comprised of ΔV
Q
(caused by the
capacitor discharge) and ΔV
ESR
(caused by the ESR of
the capacitor). Use low-ESR ceramic or aluminum elec-
trolytic capacitors at the output. For aluminum elec-
trolytic capacitors, the entire output ripple is
contributed by ΔV
ESR
. Use the ESR
OUT
equation to cal-
culate the ESR requirements and choose the capacitor
accordingly. If using ceramic capacitors, assume the
contribution to the output ripple voltage from the ESR
and the capacitor discharge are equal. Calculate the
output capacitance and ESR required for a specified
ripple using the following equations:
where
ΔI
L
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 converter.
The high switching frequency of the MAX5099 allows
for higher closed-loop bandwidth, reducing t
RESPONSE
and the output capacitance requirement. The resistive
drop across the output capacitor ESR and the capaci-
tor discharge causes a voltage droop during a step
load. Use a combination of low-ESR tantalum or poly-
mer and ceramic capacitors for better transient load
and ripple/noise performance. Keep the maximum out-
put-voltage deviation within the tolerable limits of the
electronics being powered. When using a ceramic
capacitor, assume 80% and 20% contribution 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. Controller response
time depends on the control-loop bandwidth.
Boost Converter
The MAX5099 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 values for the inductor (L
MIN
), input capacitor (C
IN
),
and output capacitor (C
OUT
) when using the converter
in boost operation.
Inductor
Choose the minimum inductor value so the converter
remains in continuous mode operation at minimum out-
put current (I
OMIN
).
where
V
D
is the forward voltage drop of the external Schottky
diode, D is the duty cycle, and V
DS
is the voltage drop
across the internal MOSFET switch. Select the inductor
with low DC resistance and with a saturation current
(I
SAT
) rating higher than the peak switch current limit of
4.3A (I
CL1
) and 2.6A (I
CL2
) of converter 1 and converter 2,
respectively.
Input Capacitor
The input current for the boost converter is continuous,
and the RMS ripple current at the input is low. Calculate
the capacitor value and ESR of the input capacitor
using the following equations:
C
I
f
V
ESR
V
I
IN
L
SW
Q
ESR
L
=
Ч
Ч
=
Δ
Δ
Δ
Δ
8
D
V
V
V
V
V
V
O
D
IN
O
D
DS
=
+
+
−
−
L
V
D
f
V
I
MIN
IN
SW
O
OMIN
=
Ч
Ч
Ч
Ч
2
2
ESR
V
I
C
I
t
V
OUT
ESR
STEP
OUT
STEP
RESPONSE
Q
=
=
×
Δ
Δ
Δ
Δ
Δ
V
V
V
O RIPPLE
ESR
Q
_
≅
+
ESR
V
I
C
I
V
f
OUT
ESR
L
OUT
L
Q
SW
=
=
Ч
Ч
Δ
Δ
Δ
Δ
8
Dual, 2.2MHz, Automotive Synchronous Buck
Converter with 80V Load-Dump Protection
______________________________________________________________________________________
19