mp1517dr MPS, mp1517dr Datasheet - Page 8

• Manufacturer Part Number: mp1517dr
• Description: 3a, 25v, 1.1mhz Step-up Converter
• Manufacturer: MPS

The DC loop gain is:
Where V
There is also a right-half-plane zero (f
exists in the continuous conduction mode
(inductor current does not drop to zero on each
cycle) step-up converters. The frequency of the
right-half plane zero is:
To stabilize the regulation control loop, the
crossover frequency (The frequency where the
loop gain drops to 0dB or a gain of 1) should be
less than half of f
75KHz. f
maximum output load current.
In some cases, an output capacitor with a high
capacitance
SEPIC CONVERTER COMPONENT
SELECTION
Selecting the Input Capacitor
An input capacitor is required to supply the AC
ripple current to the inductor, while limiting
noise at the input source. The input capacitor
selection is the same as that in the General
Purpose Component Selection section above.
Selecting the Inductors
The SEPIC converter inductors (refer to Figure
4) are required to store energy, and generate
an output voltage that is less than or greater
than the input voltage. If a coupled inductor is
used in a SEPIC converter, then the mutual
inductance of each winding forces each
inductor
inductance. Therefore smaller inductance can
be used with a coupled inductor. But the core
saturation of the coupled inductors is related to
the sum of both inductor currents.
There are two current paths to the internal
N-Channel MOSFET
converter. One is from L1 and the other is from
L2.
MP1517 Rev. 1.4
9/21/2007
A
VDC
FB
RHPZ
to
is the feedback regulation threshold.
=
f
RHPZ
7
and
is at its lowest frequency at
become
×
A
=
RHPZ
VEA
2
V
×
high
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
IN
×
π
and should be at most
2
V
V
×
switch in a SEPIC
×
IN
L
OUT
twice
×
R
×
equivalent
V
LOAD
R
2
OUT
LOAD
2
the
×
V
FB
RHPZ
© 2007 MPS. All Rights Reserved.
original
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series
) that
MP1517 – 3A, 25V, 1.1MHz STEP-UP CONVERTER
resistance (ESR) is used, then a second
compensation capacitor (from COMP to SGND)
is required to compensate for the zero
introduced by the output capacitor ESR. The
extra capacitor is required if the ESR zero is
less than 4x the crossover frequency. The ESR
zero frequency is:
If this is the case, calculate the second
compensation capacitor by the equation:
For most applications C6 is not required.
Typical
components are:
Each inductor’s ripple current can be defined
as:
Where V
∆I is the peak to peak inductor ripple current.
Set ∆I to approximately 20% of the maximum
switch current. Each inductor’s peak current is:
The total of these two currents is the total
switch current, and should be less than the
minimum device current limit of 3A.
I
L
D
I
( 1
L
PEAK
( 2
values
is the voltage drop on diode D1, and
PEAK
f
ZESR
D
)
L
)
=
C6 =
=
2
=
=
L
∆
R3 = 2.2kΩ
=
2
C3 = 10nF
V
1
I
∆
2
2
OUT
V
=
+
I
for
V
×
O
+
C2
I
OUT
f
UT
f
π
V
LOAD
SW
SW
I
+
LOAD
IN
×
×
R
×
V
C2
R
×
×
+
×
3
1 (
D
1
the
×
D
ESR
∆
V
∆
×
−
+
×
I
I
D
V
R
D)
V
V
OUT
ESR
IN
IN
V
V
compensation
+
IN
IN
+
V
V
D
D
8