MP1471 – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
MP1471 Rev. 1.01 www.MonolithicPower.com 13
8/27/2013 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
OUT OUT
OUT ESR
S1 IN S
VV
1
V1R
fL V 8fC2
⎛⎞
⎛⎞
Δ= ×− × +
⎜⎟
⎜⎟
×××
⎝⎠
⎝⎠
Where L
1
is the inductor value and R
ESR
is the
equivalent series resistance (ESR) of the output
capacitor.
For ceramic capacitors, the capacitance
dominates the impedance at the switching
frequency and causes most of the output
voltage ripple. For simplification, estimate the
output voltage ripple with:
OUT OUT
OUT
2
IN
S1
VV
V1
V
8f L C2
⎛⎞
=×−
⎜⎟
×××
⎝⎠
For tantalum or electrolytic capacitors, the ESR
dominates the impedance at the switching
frequency. For simplification, the output ripple
can be approximated with:
OUT OUT
OUT ESR
IN
S1
VV
V1R
fL V
⎛⎞
=×−×
⎜⎟
×
⎝⎠
The characteristics of the output capacitor also
affect the stability of the regulation system. The
MP1471 can be optimized for a wide range of
capacitance and ESR values.
External Bootstrap Diode
An external bootstrap (BST) diode can enhance
the efficiency of the regulator given the
following applicable conditions:
z V
OUT
is 5V or 3.3V; and
z Duty cycle is high: D=
IN
OUT
V
V
>65%
Connect the external BST diode from the output
of voltage regulator to the BST pin, as shown in
Figure 7.
Figure 7 : Optional External Bootstrap Diode
For most applications, use an IN4148 for the
external BST diode is IN4148, and a 1µF
capacitor for the BST capacitor.
PC Board Layout
PCB layout is very important to achieve stable
operation. For best results, use the following
guidelines and Figure 8 as reference.
1) Keep the connection between the input
ground and GND pin as short and wide as
possible.
2) Keep the connection between the input
capacitor and IN pin as short and wide as
possible.
3) Use short and direct feedback connections.
Place the feedback resistors and compensation
components as close to the chip as possible.
4) Route SW away from sensitive analog areas
such as FB.
C1
C6
R1
R2
R7
R6 C5
R4
C3
R5
123
456
VIN
GND
C3
R3
C1
C1A
C6
R8
R1
R2
R7
C7
R3
C3
R6 C5
R4
C4
R5
1
23
456
VIN
GND
Figure 8: Sample Layout
