Datasheet

 2014 Microchip Technology Inc. DS20005308B-page 21
MCP16331
EXAMPLE 5-4:
A 0.5A to 1A Diode is recommended.
5.8 Boost Diode
The boost diode is used to provide a charging path from
the low-voltage gate drive source while the switch node
is low. The boost diode blocks the high voltage of the
switch node from feeding back into the output voltage
when the switch is turned on, forcing the switch node
high.
A standard 1N4148 ultra-fast diode is recommended
for its recovery speed, high voltage blocking capability,
availability and cost. The voltage rating required for the
boost diode is V
IN
.
For low-boost voltage applications, a small Schottky
diode with the appropriately rated voltage can be used
to lower the forward drop increasing the boost supply
for gate drive.
5.9 Boost Capacitor
The boost capacitor is used to supply current for the
internal high side drive circuitry that is above the input
voltage. The boost capacitor must store enough energy
to completely drive the high-side switch on and off. A
0.1 µF X5R or X7R capacitor is recommended for all
applications. The boost capacitor maximum voltage is
5.5V, so a 6.3V or 10V rated capacitor is recommended.
5.10 Thermal Calculations
The MCP16331 is available in 6-lead SOT-23 and 8-
lead TDFN packages. By calculating the power
dissipation and applying the package thermal
resistance (

JA
), the junction temperature is estimated.
To quickly estimate the internal power dissipation for
the switching step-down regulator, an empirical calcu
-
lation using measured efficiency can be used. Given
the measured efficiency, the internal power dissipation
is estimated by
Equation 5-7. This power dissipation
includes all internal and external component losses.
For a quick internal estimate, subtract the estimated
Schottky diode loss and inductor DCR loss from the
P
DIS
calculation in Equation 5-7.
EQUATION 5-7: TOTAL POWER
DISSIPATION ESTIMATE
The difference between the first term, input power, and
the second term, power delivered, is the total system
power dissipation. The freewheeling Schottky diode
losses are determined by calculating the average diode
current and multiplying by the diode forward drop. The
inductor losses are estimated by P
L
= I
OUT
2
x L
DCR
.
EQUATION 5-8: DIODE POWER
DISSIPATION ESTIMATE
EXAMPLE 5-5:
TABLE 5-5: FREEWHEELING DIODES
App Manufacturer
Part
Number
Rating
12 V
IN
500 mA
Diodes 
Inc.
DFLS120L-7 20V, 1A
24 V
IN
100 mA
Diodes 
Inc.
B0540Ws-7 40V, 0.5A
18 V
IN
500 mA
Diodes 
Inc.
B130L-13-F 30V, 1A
48 V
IN
500 mA
Diodes 
Inc.
B1100 100V, 1A
I
OUT
=0.5A
V
IN
= 15V
V
OUT
=5V
D=5/15
I
DAVG
= 333 mA
V
OUT
I
OUT

Efficiency
-------------------------------


V
OUT
I
OUT
 – P
Dis
=
P
Diode
V
F
1D–I
OUT
   =
V
IN
=10V
V
OUT
=5.0V
I
OUT
=0.4A
Efficiency = 90%
Total System Dissipation = 222 mW
L
DCR
=0.15
P
L
=24 mW
Diode V
F
=0.50
D=50%
P
Diode
=125 mW
MCP16331 internal power dissipation estimate:
P
DIS
- P
L
- P
DIODE
= 73 mW

JA
=198°C/W
Estimated Junction 
Temperature Rise
=+14.5°C