Rainbow Electronics MAX8514 User Manual
Page 19
MAX8513/MAX8514
Wide-Input, High-Frequency, Triple-Output Supplies
with Voltage Monitor and Power-On Reset
______________________________________________________________________________________
19
diode on the secondary side (Q2 and D2 in the Typical
Applications Circuits). n
2
and n
1
are the number of
turns of the secondary winding and the primary wind-
ing, respectively.
It is important to have the secondary winding tightly
coupled with the primary winding to minimize leakage
inductance for higher efficiency. The positive voltage
generated by the secondary winding can also be
stacked with the main DC-DC step-down converter out-
put to further improve efficiency and reduce winding
cost. In this case, the secondary-side voltage is:
Input Capacitor
The input-filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the AC-RMS current
through the ESR of the input capacitor (C2 in the Typical
Applications Circuits). The input capacitor must meet
the ripple-current requirement (I
IN_RMS
) imposed by the
switching currents defined by the following equation:
I
IN_RMS
has a maximum value when the input voltage
equals twice the output voltage (V
IN
= 2 × V
OUT1
), so
I
IN_RMS(MAX)
= I
OUT1
/ 2. Ceramic capacitors are rec-
ommended due to their low ESR and ESL at high fre-
quency, with relatively low cost. Choose a capacitor that
exhibits less than 10°C temperature rise at the maximum
operating RMS current for optimum long-term reliability.
For applications that require input power-fail warning,
such as dying gasp, add a large-value electrolytic
capacitor (C
S
) to the input as a local energy storage
device to provide the power to the converter in case of
input power-fail. The capacitor value must be high
enough to meet the desired power-fail warning time,
t
WARN
, where t
WARN
is the time from when PFI trips the
PFO output to when the main output (OUT1) starts
dropping out of regulation. The value of the storage
capacitor, C
S
, can be calculated as:
where P
OUT1
is the total output power, η is the total
converter efficiency, V
PFI
is the input voltage value at
the input power-fail (PFI) trip threshold, and V
DROOP
is
the input voltage value where V
OUT1
starts dropping
out of regulation.
V
PFI
and V
DROOP
can be calculated as:
where R10 and R11 are the resistive-dividers from IN to
PFI to GND in the Typical Applications Circuits.
where D
MAX
is the maximum duty cycle.
To ensure for worst-case component tolerances such
as capacitance of C
S
, converter efficiency, V
PFI
, and
V
DROOP
’s threshold over the operating temperature
range, it is recommended to select C
S
at least 1.5 times
the calculated value above.
Output Capacitor
The key selection parameters for the output capacitor
are the actual capacitance value, the equivalent series
resistance (ESR), the equivalent series inductance
(ESL), and the voltage-rating requirements. All of these
affect the overall stability, output ripple voltage, and
transient response.
The output ripple is composed of three components: vari-
ations in the charge stored in the output capacitor, the
voltage drop across the capacitor’s equivalent series
resistance (ESR), and equivalent series inductance (ESL)
caused by the current into and out of the capacitor.
V
V
D
DROOP
OUT
MAX
=
1
V
V
R
R
PFI
.
=
×
+
1 22
1
10
11
C
P
t
V
S
OUT
WARN
PFI
.
=
Ч
Ч
Ч
(
)
0 5
1
η
1
V
-
1
V
- V
PFI
DROOP
DROOP
I
I
V
V
V
V
IN RMS
OUT
OUT
IN
OUT
IN
_
=
Ч
Ч
(
)
1
1
1
-
V
V
V
n
n
V
SEC
OUT
Q
OUT
=
+
(
)
×
1
2
2
1
1
+
- V
D2