MCP1650/51/52/53
DS21876B-page 18 2004-2013 Microchip Technology Inc.
To determine the maximum inductance for
Discontinuous Operating mode, multiply the energy
going into the inductor every switching cycle by the
number of cycles per second (switching frequency).
This number must be greater than the maximum input
power.
The equation for the energy flowing into the inductor is
given below. The input power to the system is equal to
energy times time.
The inductor peak current is calculated using the
equation below:
Using a typical inductance of 3.3 µH, the peak current
in the inductor is calculated below:
At 3.8V and below, the converter can boost to 14V
while operating in the Continuous mode.
For this example, a 3.3 µH inductor is too large, a
2.2 µH inductor is selected.
As the inductance is lowered, the peak current drawn
from the input at all loads is increased. The best choice
of inductance for high boost ratios is the maximum
inductance value necessary while maintaining
discontinuous operation.
For lower boost-ratio applications (3.3V to 5.0V), a
3.3 µH inductor or larger is recommended. In these
cases, the inductor operates in Continuous Current
mode.
5.2.2 MOSFET SELECTION
There are a couple of key consideration’s when
selecting the proper MOSFET for the boost design. A
low R
DSON
logic-level N-channel MOSFET is
recommended.
5.2.2.1 MOSFET Selection Process.
1. Voltage Rating - The MOSFET drain-to-source
voltage must be rated for a minimum of V
OUT
+
V
FD
of the external boost diode. For example, in
the 12V output converter, a MOSFET drain-to-
source voltage rating of 12V + 0.5V is
necessary. Typically, a 20V part can be used for
12V outputs.
2. Logic-Level R
DSON
- The MOSFET carries
significant current during the boost cycle on
time. During this time, the peak current in the
MOSFET can get quite high. In this example, a
SOT-23 MOSFET was used with the following
ratings:
Selecting MOSFETs with lower R
DSON
is not always
better or more efficient. Lower R
DSON
typically results
in higher total gate charge and input capacitance, slow-
ing the transition time of the MOSFET and resulting in
increased switching losses.
5.2.3 DIODE SELECTION
The external boost diode also switches on and off at the
switching frequency and requires very fast turn-on and
turn-off times. For most applications, Schottky diodes
are recommended. The voltage rating of the Schottky
diode must be rated for maximum boost output voltage.
For example, 12V output boost converter, the diode
should be rated for 12V plus margin. A 20V or 30V
Schottky diode is recommended for a 12V output appli-
cation. Schottky diodes also have low forward-drop
characteristics, another desired feature for switching
power supply applications.
F
SW
=750kHz
T
ON
=(1/F
SW
* Duty Cycle)
I
PK
(2.8V) = 905 mA
Energy (2.8V) = 1.35 µ-Joules
Power (2.8V) = 1.01 Watts
I
PK
(3.8V) = 860 mA
Energy at 3.8V = 1.22 µ-Joules
Power = 0.914 Watts
F
SW
= 750 kHz
T
ON
=(1/F
SW
* Duty Cycle)
I
PK
(2.8V) = 1.36A
Energy (2.8V) = 2.02 µ-Joules
Power (2.8V) = 1.52 Watts
I
PK
(3.8V) = 1.29A
Energy at 3.8V = 1.83 µ-Joules
Power = 1.4 Watts
Energy
1
2
-- - LI
PK
2
=
I
PK
V
IN
L
-------- T
ON
=
IRLM2502 N-channel MOSFET
V
BDS
= 20V (Drain Source Breakdown
Voltage)
R
DSON
= 50 milli-ohms (V
GS
= 2.5V)
R
DSON
= 35 milli-ohms (V
GS
= 5.0V)
Q
G
= Total Gate Charge = 8 nC
V
GS
= 0.6V to 1.2V (Gate Source Threshold
Voltage)
