MCP16311SLASH2 datasheet(21/40 Pages) MICROCHIP

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DS20005255C-page 21

MCP16311/2

5.7

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 100 nF

X5R or X7R capacitor is recommended for all

applications. The boost capacitor maximum voltage is 5V.

5.8

Vcc Capacitor

The VCC internal bias regulates at 5V. The VCC pin is

current limited to 50 mA and protected from a short-

circuit condition at 150 mA load. The VCC regulator

must sustain all load and line transients because it

supplies the internal drivers for power switches. For

stability reasons, the VCC capacitor must be at least

1 µF X7R ceramic for extended temperature range, or

X5R for limited temperature range.

5.9

MCP16312 – LED Constant

Current Driver

MCP16312 can be used to drive an LED or a string of

LEDs. The process of transforming the MCP16312

from a constant voltage source into a constant current

source is simple. It implies that the sensing/feedback

for the current is on the low side by adding a resistor in

series with the string of LEDs.

When using the MCP16312 as an LED driver, care must

be taken when selecting the sense resistor. Due to the

high feedback voltage of 0.8V, there will be significant

losses on the sense resistor, so a larger package with

better power dissipation must be selected.

Another important aspect when creating such an

application is the value of the inductor. The value of the

inductor needs to follow Equation 5-3 or, as a guideline,

Table 5-1, where the output voltage is approximated as

the sum of the forward voltages of the LEDs and a 0.8V

headroom for the sense resistor. A typical application is

shown in Figure 5-3.

The following equations are used to determine the

value and the losses for the sense resistor:

EQUATION 5-6:

EXAMPLE 5-5:

5.10

Thermal Calculations

The MCP16311/2 is available in MSOP-8 and DFN-8

packages. By calculating the power dissipation and

applying the package thermal resistance (θJA), the

junction temperature is estimated. The maximum

continuous junction temperature rating for the

MCP16311/2 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 in Equation 5-7. This power

dissipation

includes

all

internal

and

external

component losses. For a quick internal estimate,

subtract the estimated inductor DCR loss from the PDIS

calculation in Equation 5-7.

EQUATION 5-7:

TOTAL POWER

DISSIPATION ESTIMATE

Wurth Elektronik®

74408943150

0.118

1.7

4.8x4.8x3.8

744062150

0.085

1.1

6.8x6.8x2.3

744778115

0.1

1.75

7.3x7.3x3.2

7447779115

0.07

2.2

7.3x7.3x4.5

Coiltronics®

SD25

0.095 1.08

5.2x5.2x2.5

SD6030

14.1 0.103

1.1

6.0x6.0x3.0

TDK - EPC®

B82462G4153M

0.097 1.05

6.0x6.0x3.0

B82462A4153K

0.21

1.5

6.0x6.0x3.0

TABLE 5-3:

MCP16311/2 RECOMMENDED

3.3V VOUT INDUCTORS

Part Number

Size

WxLxH

(mm)

ILED = 400 mA

VFB =0.8V

VF = 1 x 3.2V (one white LED is used)

RB =2

PLOSSES = 0.32 W (sense resistor losses)

L= 22 µH

VFB

ILED

-----------

PLOSSES

VFB ILED



Where:

VFB = Feedback Voltage

PDIS

VOUT IOUT



Efficiency

-------------------------------VOUT IOUT





–

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