mp1517dr MPS, mp1517dr Datasheet - Page 7

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

Use an input capacitor value greater than 10µF.
The capacitor can be electrolytic, tantalum or
ceramic. However since it absorbs the input
switching current it requires an adequate ripple
current rating. Use a capacitor with RMS current
rating greater than the inductor ripple current.
To insure stable operation place the input
capacitor as close to the IC as possible.
Alternately a smaller high quality ceramic 0.1µF
capacitor may be placed closer to the IC with the
larger capacitor placed further away. If using this
technique, it is recommended that the larger
capacitor be a tantalum or electrolytic type. All
ceramic capacitors should be placed close to the
MP1517.
Selecting the Output Capacitor
The output capacitor is required to maintain the
DC output voltage. Low ESR capacitors are
preferred to keep the output voltage ripple to a
minimum. The characteristic of the output
capacitor also affects the stability of the regulation
control system. Ceramic, tantalum, or low ESR
electrolytic capacitors are recommended. In the
case of ceramic capacitors, the impedance of the
capacitor at the switching frequency is dominated
by the capacitance, and so the output voltage
ripple is mostly independent of the ESR. The
output voltage ripple is calculated as:
Where V
V
respectively, I
switching frequency, and C2 is the capacitance of
the output capacitor.
In the case of tantalum or low-ESR electrolytic
capacitors, the ESR dominates the impedance at
the switching frequency, and so the output ripple
is calculated as:
Where R
the output capacitors.
Choose an output capacitor to satisfy the output
ripple and load transient requirements of the
MP1517 Rev. 1.4
9/21/2007
V
OUT
RIPPLE
are the DC input and output voltages
=
RIPPLE
ESR
I
V
LOAD
RIPPLE
is the equivalent series resistance of
LOAD
is the output ripple voltage, V
×
⎡
⎢
⎣
V
=
is the load current, f
OUT
(
I
V
LOAD
V
OUT
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
O
×
UT
f
×
SW
−
(
×
V
V
C
IN
×
O
UT
2
C
)
×
2
−
f
+
SW
V
R
IN
ESR
)
V
SW
© 2007 MPS. All Rights Reserved.
×
IN
V
www.MonolithicPower.com
IN
is the
OUT
and
⎤
⎥
⎦
MP1517 – 3A, 25V, 1.1MHz STEP-UP CONVERTER
design. Place the output capacitor close to SW
to minimize the AC loop and switching noise.
Selecting the Diode
The output rectifier diode supplies current to the
inductor when the internal MOSFET is off. To
reduce losses due to diode forward voltage and
recovery time, use a Schottky diode. Choose a
diode whose maximum reverse voltage rating is
greater than the maximum output voltage. The
rated average forward current needs to be
equal to or greater than the load current.
Selecting the Soft-Start Capacitor
The soft-start period is determined by the equation:
Where C
from SS to SGND, and t
soft-start period.
Determine the capacitor required for a given
soft-start period by the equation:
It is recommended that values between 10nF and
22nF for C
Compensation
The output of the transconductance error
amplifier (COMP) is used to compensate the
regulation control system. The system uses two
poles and one zero to stabilize the control loop.
The poles are f
and load resistance and f
compensation capacitor C3. The zero f
by the compensation capacitor C3 and the
compensation resistor R3.
These are determined by the equations:
Where R
error amplifier transconductance, and A
the error amplifier voltage gain.
LOAD
SS
SS
(in nF) is the soft-start capacitor
be used to set the soft-start period.
f
P2
is the load resistance, G
f
f
P1
Z1
=
P1
t
C4
=
SS
=
2
set by the output capacitor
π
2
=
×
=
×
×
0.275
π
3.64
C2
π
×
G
×
C3
×
C3
1
1
EA
×
R
×
×
t
SS
C4
SS
LOAD
×
A
R3
VEA
P2
(in ms) is the
set by the
EA
Z1
VEA
is the
is set
is
7