MP1494 – SYNCHRONOUS STEP-DOWN CONVERTER
MP1494 Rev. 1.04 www.MonolithicPower.com 13
12/26/2012 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
External Bootstrap Diode
An external bootstrap diode can enhance the
efficiency of the regulator given the following
conditions:
z V
OUT
is 5V or 3.3V; and
z Duty cycle is high: D=
IN
OUT
V
V
>65%
In these cases, add an external BST diode from
the VCC pin to BST pin, as shown in Figure 7.
SW
BST
MP1494
C
L
BST
C
OUT
External BST Diode
VCC
IN4148
Figure 7: Optional External Bootstrap Diode to
Enhance Efficiency
The recommended external BST diode is
IN4148, and the BST capacitor value is 0.1µF
to 1F.
PC Board Layout
(8)
PCB layout is very important to achieve stable
operation especially for VCC capacitor and
input capacitor placement. For best results,
follow these guidelines:
1) Use large ground plane directly connect to
GND pin. Add vias near the GND pin if bottom
layer is ground plane.
2) Place the VCC capacitor to VCC pin and
GND pin as close as possible. Make the trace
length of VCC pin-VCC capacitor anode-VCC
capacitor cathode-chip GND pin as short as
possible.
3) Place the ceramic input capacitor close to IN
and GND pins. Keep the connection of input
capacitor and IN pin as short and wide as
possible.
4) Route SW, BST away from sensitive analog
areas such as FB. It’s not recommended to
route SW, BST trace under chip’s bottom side.
5) Place the T-type feedback resistor R9 close
to chip to ensure the trace which connects to
FB pin as short as possible
Notes:
8) The recommended layout is based on the Figure 8 Typical
Application circuit on the next page.
8
7
6
5
L1
C2
C2A
C1
C1A
R5R6
R7
R9
R8
R1
R2
R3
R4
C3
C4
C5
C6
1
2
3
4
Vin
GND
Vout
SW
GND
GND
SW
GND
EN/SYNC
BST
VCC
