MCP16331 datasheet(17/48 Pages) MICROCHIP

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DS20005308D-page 17

MCP16331

Shunt boost supply regulation is used for low output

voltage converters operating from a wide ranging input

source; a regulated 3.0V to 5.5V supply is needed to

provide high-side drive bias. The shunt uses a Zener

diode to clamp the voltage within the 3.0V to 5.5V

range, using the RSH resistor shown in Figure 4-3.

To calculate the RSH resistor value, the boost drive

current needs to be estimated first, using Equation 4-1.

IBOOST_TYP for 3.3V Boost Supply = 0.6 mA

IBOOST_TYP for 5.0V Boost Supply = 0.8 mA.

EQUATION 4-1:

BOOST CURRENT

To calculate the RSH resistor value, the maximum

IBOOST and IZ current are used at the minimum input

voltage (Equation 4-2).

EQUATION 4-2:

SHUNT RESISTANCE

VZ and IZ can be found on the Zener diode

manufacturer’s data sheet; typically, IZ = 1 mA.

Series regulator applications use a Zener diode to drop

the excess voltage; the series regulator bias source

can be input or output voltage derived, as shown in

Figure 4-4. For proper circuit operation, the boost sup-

ply must remain between 3.0V and 5.5V at all times.

FIGURE 4-4:

MCP16331 Series Regulator Boost Supply.

IBOOST = IBOOST_TYP 1.5 mA

RSH

VINMIN VZ

–

IBoost IZ

------------------------------

VOUT

VIN

CIN

COUT

BOOST

GND

RTOP

RBOT

VIN

Boost Diode

FW Diode

12V

15V to 50V

VIN

CIN

BOOST

GND

VIN

Boost Diode

FW Diode

12V

VZ =7.5V

VOUT

RTOP

RBOT

COUT

MCP16331

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