Microchip Technology ADM00352 Data Sheet

DS20005004C-page 20

 2011-2013 Microchip Technology Inc.

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.

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. In case of a noise-sensitive application,
an additional resistor in series with the boost capacitor,
that will reduce the high-frequency noise associated
with switching power supplies, can be added. A typical
value for the resistor is 82

.

The MCP16301/H devices are available in a SOT-23-6
package. By calculating the power dissipation and
applying the package thermal resistance (



JA

), the

junction temperature is estimated. The maximum
continuous junction temperature rating for the
MCP16301/H devices is +125°C.

To quickly estimate the internal power dissipation for
the switching step-down regulator, an empirical
calculation using measured efficiency can be used.
Given the measured efficiency, the internal power
dissipation is estimated by 

. This power

dissipation includes all internal and external
component losses. For a quick internal estimate,
subtract the estimated Schottky diode loss and inductor
ESR loss from the P

DIS

 calculation in 

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

ESR

.

Good printed circuit board layout techniques are
important to any switching circuitry, and switching
power supplies are no different. When wiring the
switching high-current paths, short and wide traces
should be used. Therefore, it is important that the input
and output capacitors be placed as close as possible to
the MCP16301/H devices to minimize the loop area.

The feedback resistors and feedback signal should be
routed away from the switching node and the switching
current loop. When possible, ground planes and traces
should be used to help shield the feedback signal and
minimize noise and magnetic interference.

A good MCP16301/H layout starts with C

IN

 placement.

C

IN

 supplies current to the input of the circuit when the

switch is turned on. In addition to supplying
high-frequency switch current, C

IN

 also provides a

stable voltage source for the internal MCP16301/H
circuitry. Unstable PWM operation can result if there
are excessive transients or ringing on the V

IN

 pin of the

MCP16301/H devices. In 

, C

IN

 is placed

close to pin 5. A ground plane on the bottom of the
board provides a low resistive and inductive path for
the return current. The next priority in placement is the
freewheeling current loop formed by D1, C

OUT

 and L,

while strategically placing C

OUT

 return close to C

IN

return. Next, C

B

 and D

B

 should be placed between the

boost pin and the switch node pin SW. This leaves
space close to the V

FB

 pin of the MCP16301/H devices

to place R

TOP

 and R

BOT

. R

TOP

 and R

BOT

 are routed

away from the Switch node so noise is not coupled into
the high-impedance V

FB

 input.

















–

–



 I









V

IN

= 10V

V

OUT

= 5.0V

I

OUT

= 0.4A

Efficiency

= 90%

Total System Dissipation

= 222 mW

L

ESR

= 0.15



P

L

= 24 mW

Diode VF

= 0.50

D

= 50%

P

Diode

= 125 mW

MCP16301/H internal power dissipation estimate:

P

DIS

- P

L

- P

DIODE

= 73 mW



JA

= 198°C/W

Estimated Junction 
Temperature Rise

= +14.5°C