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MAX17497A View Datasheet(PDF) - Maxim Integrated

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MAX17497A Datasheet PDF : 30 Pages
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MAX17497A/MAX17497B
AC-DC and DC-DC Peak Current-Mode Converters
with Integrated Step-Down Regulator
diode current) should be low enough to ensure that the
junction temperature is within limits. This necessitates
the diode-current rating be in the order of 2 x IOUTF to
3x IOUTF. Select fast-recovery diodes with a recovery
time less than 50ns, or Schottky diodes with low junction
capacitance.
Error-Amplifier Compensation Design
The loop-compensation values are calculated as:
R=Z 450 ×
1+
0.1fSW
fP
2
×
VOUTF
×
IOUTF
2 × LPRI × fSW
fP
=
π×
IOUTF
VOUTF × COUTF
CZ
=
π
1
×RZ
× fP
CP
=
π
1
×RZ ×
fSW
The devices’ switching frequency (fSW) can be ob-
tained from the Electrical Characteristics section.
In a typical application, the integrated step-down
regulator is fed off the flyback converter’s output. The
step-down regulator poses negative input impedance
or constant input power behavior. Due to this behavior,
the loop bandwidth measured for the flyback converter
would be smaller than the design bandwidth.
CCM Flyback
Transformer Turns Ratio Calculation (K = Ns/Np)
The transformer turns ratio can be calculated using the
following equation:
K = (VOUT + VD) × (1DMAX)
VINMIN × DMAX
where DMAX is the duty cycle assumed at minimum input
(0.35 for the MAX17497A and 0.7 for the MAX17497B).
Primary Inductance Calculation
Calculate the primary inductance based on the ripple:
L PRI
=
(VOUTF + VD) × (1DNOM)2
2 × IOUTF × β × fSW × K 2
where DNOM, the nominal duty cycle at nominal operat-
ing DC input voltage (VINNOM), is given as:
DNOM
=
(VOUT + VD)
VINNOM × K + (VOUT
+
VD)
The output current, down to which the flyback converter
should operate in CCM, is determined by selection of
the fraction A in the above primary inductance formula.
For example, A should be selected as 0.15 so that the
converter operates in CCM down to 15% of the maxi-
mum output-load current. Since the ripple in the primary
current waveform is a function of duty cycle, and is
maximum-at-maximum DC input voltage, the maximum
(worst-case) load current, down to which the converter
operates in CCM, occurs at maximum operating DC input
voltage. VD is the forward drop of the selected output
diode at maximum output current.
Peak and RMS Current Calculation
RMS current values in the primary and secondary are
needed by the transformer manufacturer to design the
wire diameter for the different windings. Peak-current
calculations are useful in setting the current limit. Use the
following equations to calculate the primary and second-
ary peak and RMS currents:
Maximum primary peak current:
= IPRIPEAK
I1OUDTFM×AXK
+
VINMIN ×
2 × LPRI
DMAX
× fSW
Maximum primary RMS current:
= IPRIRMS
IPRIPEAK
2
+

IPRI2
3

(IPRIPEAK
×
IPRI)
× DMAX 2
where DIPRI is the ripple current in the primary current
waveform, and is given by:
IPRI
=  VINLMPIRNI
×
×
DMAX
fSW
Maximum secondary peak current:
ISECPEAK
=
IPRIPEAK
K
Maximum secondary RMS current:
Maxim Integrated
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