LPS3015 View Datasheet(PDF) - Microchip Technology
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LPS3015 Datasheet PDF : 38 Pages
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MCP16301
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 recom-
mended.
5.10 Thermal Calculations
The MCP16301 is available in a SOT-23-6 package. By
calculating the power dissipation and applying the
package thermal resistance (θJA), the junction temper-
ature is estimated. The maximum continuous junction
temperature rating for the MCP16301 is +125°C.
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 ESR loss from the
PDIS calculation in Equation 5-7.
EQUATION 5-7: TOTAL POWER
DISSIPATION ESTIMATE
⎛
⎝
V---E-O---f-U-f--iT--c--×-i--e--I-n-O--c--U-y--T- ⎠⎞
– (VOUT × IOUT)
=
PDis
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 PL = IOUT2 x LESR.
EQUATION 5-8: DIODE POWER
DISSIPATION ESTIMATE
PDiode = VF × ((1 – D) × IOUT)
EXAMPLE 5-5:
VIN
VOUT
IOUT
Efficiency
Total System Dissipation
LESR
PL
Diode VF
D
PDiode
= 10V
= 5.0V
= 0.4A
= 90%
= 222 mW
= 0.15Ω
= 24 mW
= 0.50
= 50%
= 125 mW
MCP16301 internal power dissipation estimate:
PDIS - PL - PDIODE = 73 mW
θJA
Estimated Junction
Temperature Rise
= 198°C/W
= +14.5°C
5.11 PCB Layout Information
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 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 layout starts with CIN placement.
CIN supplies current to the input of the circuit when the
switch is turned on. In addition to supplying high-
frequency switch current, CIN also provides a stable
voltage source for the internal MCP16301 circuitry.
Unstable PWM operation can result if there are
excessive transients or ringing on the VIN pin of the
MCP16301 device. In Figure 5-1, CIN 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, COUT and L,
while strategically placing COUT return close to CIN
return. Next, CB and DB should be placed between the
boost pin and the switch node pin SW. This leaves
space close to the MCP16301 VFB pin to place RTOP
and RBOT. RTOP and RBOT are routed away from the
Switch node so noise is not coupled into the high-
impedance VFB input.
DS25004A-page 20
© 2011 Microchip Technology Inc.
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