mp1517dr MPS, mp1517dr Datasheet - Page 9

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

Selecting the Output Diode
The output diode is typically a Schottky diode.
The Schottky diode is selected based upon
voltage requirement, current rating, and thermal
capability. The diode breakdown and power
switch breakdown voltage are set higher than
V
both of these devices. The current rating is set
based on the average load current in the diode.
The average diode current is equal to the load
current, but the peak current and power
consumption on D1 is:
Selecting the Coupling Capacitor
The steady state voltage across L1 and L2 is
equal to zero. Therefore the coupling capacitor
has V
capacitor will need to be rated for an input
voltage plus some guard band. Also this
capacitor will need an I
Application Examples
Figure 2 shows a typical application circuit
driving multiple strings of LEDs with the
MP1517. The 3 strings of 6 white LEDs can be
driven from a voltage supply range of 2.6V to
6V at an output current of 20mA. A 1µF output
capacitor is usually sufficient for this kind of
application. A 4.7µH inductor with low DCR
(inductor
improve efficiency. A 10µF ceramic capacitor is
recommended
Schottky diodes have fast recovery and a low
forward voltage and are recommended. The
MP1517 soft-start helps to limit the amount of
current through V
the amount of overshoot on the output.
MP1517 Rev. 1.4
9/21/2007
OUT
+ V
IN
across it in steady state. The coupling
I
IN
I
D
C
, since this is the voltage stress on
( 1
( 8
resistance)
PEAK
RMS
)
)
P
for
=
=
D
IN
1
I
∆
2
LOAD
=
at startup and to also limit
I
+
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
I
the
LOAD
RMS
I
LOAD
×
is
ripple current rating:
×
V
input
×
V
recommended
OUT
D
V
V
IN
IN
V
+
+
IN
V
capacitance.
V
D
D
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www.MonolithicPower.com
to
MP1517 – 3A, 25V, 1.1MHz STEP-UP CONVERTER
Figure 3 shows a typical application running the
MP1517 in flash mode. During preview mode,
resistor R2 sets the current through the white
LEDs to 20mA. When a flash is required the
N-Channel MOSFET Q1 is turned on and
150mA
compensation capacitor has been increased to
47nF. This forces the compensation node to
slowly rise when Q1 is turned on which allows
the current through the inductor to slowly
increase without overshoot. By doing this the
inrush current on the input is minimized.
Figure 4 shows a SEPIC circuit using the
MP1517 to generate 3.3V output from a 3V to
4.2V input. A peak voltage detect circuit (D2
and C7) is added to the IN pin from the highest
voltage potential (SW node) in the entire circuit.
The average voltage on C7 is roughly the sum
of V
of the internal MOSFET switch. A transformer
with two windings can be used to replace the
two separate inductors in Figure 4. In this case,
the effective inductance value is doubled due to
the mutual inductance of each winding. This
reduces
improves efficiency. Figures 5 and 6 show
some other SEPIC application circuits.
Layout Consideration
High frequency switching regulators require
very careful layout for stable operation and low
noise. All components must be placed as close
to the IC as possible. Keep the path between
L1, D1, and C2 (also L2, C8 if applicable)
extremely short for minimal noise and ringing.
C1 must be placed close to the IN pin for best
decoupling. All feedback components must be
kept close to the FB pin to prevent noise
injection on the FB pin trace. The ground return
of C1 and C2 should be tied close to the PGND
pins. See the MP1517 demo board layout for
reference.
IN
and V
flows
the
OUT
. This ensures the enhancement
inductor
through
ripple
the
current
LEDs.
The
and
9