mp1517dr MPS, mp1517dr Datasheet - Page 6

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

Internal Low-Dropout Regulator
The internal power to the MP1517 is supplied
from the IN pin through an internal 2.4V low-
dropout linear regulator, whose output is BP.
Bypass BP to SGND with a 10nF or greater
capacitor to insure the MP1517 operates
properly. The internal regulator can not supply
any more current than is required to operate the
MP1517, therefore do not apply any external
load to BP.
Soft-Start
The MP1517 includes a soft-start timer that limits
the voltage at COMP during startup to prevent
excessive current at the input. This prevents
premature termination of the source voltage at
startup due to input current overshoot. When
power is applied to the MP1517, and enable is
asserted, a 2µA internal current source charges
the external capacitor at SS. As the capacitor
APPLICATION INFORMATION
GENERAL PURPOSE COMPONENT
SELECTION
Setting the Output Voltage
Set the output voltage by selecting the resistive
voltage divider ratio. Use 10kΩ to 50kΩ for the
low-side resistor R2 of the voltage divider.
Determine the high-side resistor R1 by the
equation:
where V
For R2 = 10kΩ and V
R1 (kΩ) = 14.29kΩ (V
Selecting the Inductor
The inductor is required to force the higher
output voltage while being driven by the input
voltage. A larger value inductor results in less
ripple current, resulting in lower peak inductor
current and reducing stress on the internal
N-Channel.
larger value inductor has a larger physical size,
higher
saturation current.
Choose an inductor that does not saturate
under the worst-case load transient and startup
MP1517 Rev. 1.4
9/21/2007
OUT
series
is the output voltage.
R
MOSFET switch. However, the
1
=
R
resistance,
2
×
FB
OUT
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
(
V
= 0.7V, then
OUT
V
– 0.7V).
FB
-
V
FB
and/or
)
© 2007 MPS. All Rights Reserved.
www.MonolithicPower.com
lower
MP1517 – 3A, 25V, 1.1MHz STEP-UP CONVERTER
charges, the voltage at SS rises. The MP1517
internally clamps the voltage at COMP to 700mV
above the voltage at SS. This limits the inductor
current at startup, forcing the input current to rise
slowly to the current required to regulate the
output voltage.
Open Load Shutdown
The MP1517 includes an open load detect that
will stop the output from switching. In a fault
condition where the connection to the LED’s is
open, V
the MP1517 will stop switching and the output
will stop rising. When the output falls below 27V
the MP1517 will restart in soft-start mode and
switches until the OLS threshold is exceeded
again. This will continue until the part is
disabled. To disable the open load shutdown
feature, connect the OLS pin to GND.
conditions. A good rule for determining the
inductance is to allow the peak-to-peak ripple
current to be approximately 30% to 50% of the
maximum input current. Make sure that the
peak inductor current is below 3A to prevent
loss of regulation due to the current limit.
Calculate the required inductance value by the
equation:
Where V
switching frequency, I
load current, ∆I is the peak-to-peak inductor ripple
current and η is the efficiency.
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. A low ESR capacitor
is required to keep the noise at the IC to a
minimum. Ceramic capacitors are preferred, but
tantalum or low-ESR electrolytic capacitors may
also suffice.
OUT
IN
will rise up. Once V
I
IN
∆
is the input voltage,
I
(
L
MAX
=
=
(
30
)
V
=
V
IN
%
OUT
V
×
−
OUT
(V
LOAD(MAX)
50
×
OUT
V
f
×
%
SW
IN
I
LOAD
)
×
×
-
×
I
η
V
IN
∆
OUT
IN
(
(
is the maximum
MAX
MAX
I
)
exceeds 27V,
)
)
f
SW
is the
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