MP1470 – SYNCHRONOUS, STEP-DOWN CONVERTER WITH INTERNAL MOSFETS
For most applications, use an IN4148 for the
external BST diode is IN4148, and a 1µF
capacitor for the BST capacitor.
⎛
⎞ ⎛
VOUT
⎞
⎟
⎠
VOUT
1
ΔVOUT
=
× 1−
× R
⎟ ⎜
+
⎜
ESR
fS ×L1
V
8× fS ×C2
⎝
IN ⎠ ⎝
Where L1 is the inductor value and RESR is the
equivalent series resistance (ESR) of the output
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.
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:
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.
⎛
⎞
⎟
⎠
VOUT
8× fS2 ×L1 ×C2
VOUT
ΔVOUT
=
× 1−
⎜
V
⎝
IN
For tantalum or electrolytic capacitors, the ESR
dominates the impedance at the switching
frequency. For simplification, the output ripple
can be approximated with:
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.
VOUT
VOUT
⎛
⎞
ΔVOUT
=
× 1−
×RESR
⎜
⎟
⎠
fS ×L1
V
IN
⎝
The characteristics of the output capacitor also
affect the stability of the regulation system. The
MP1470 can be optimized for a wide range of
capacitance and ESR values.
C1
GND
C6
VIN
C3
3
4
2
5
1
6
R5
R4
C5
L1
External Bootstrap Diode
An external bootstrap (BST) diode can enhance
the efficiency of the regulator given the
following applicable conditions:
R2
R1
R7
C3
R3
R6
VOUT
C2
z VOUT is 5V or 3.3V; and
C2A
VOUT
z Duty cycle is high: D=
>65%
VIN
C1A
Connect the external BST diode from the output
of voltage regulator to the BST pin, as shown in
Figure 7
C1
C6
GND
VIN
C4
R4
External BST Diode
IN4148
1
6
3
2
5
R4
R5
BST
SW
L1
4
C7
MP1470
R2
R1
R8
R6
C5
R7
C3
R3
5V or 3.3V
L
C2
C
OUT
C2A
Figure 7: Optional External Bootstrap Diode
Figure 8: Sample Board Layout
MP1470 Rev. 1.02
8/27/2013
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12
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