Datasheet

MCP16331
DS20005308C-page 20  2014-2016 Microchip Technology Inc.
5.7 Freewheeling Diode
The freewheeling diode creates a path for inductor
current flow after the internal switch is turned off. The
average diode current is dependent upon the output
load current at duty cycle (D). The efficiency of the con
-
verter is a function of the forward drop and speed of the
freewheeling diode. A low forward drop Schottky diode
is recommended. The current rating and voltage rating
of the diode is application-dependent. The diode
voltage rating should be a minimum of V
IN
plus margin.
The average diode current can be calculated using
Equation 5-6.
EQUATION 5-6: DIODE AVERAGE
CURRENT
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 the 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 the 6-lead SOT-23 and
8-lead TDFN packages. By calculating the power dissi
-
pation 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
TABLE 5-5: FREEWHEELING DIODES
App Mfr.
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
DAVG
1D–I
OUT
 =
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
   =