MCP16251_16 datasheet(17/32 Pages) MICROCHIP

Part #	MCP16251

Description	Low Quiescent Current, PFM/PWM Synchronous Boost Regulator with True Output Disconnect or Input/Output Bypass Option

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 2013 - 2016 Microchip Technology Inc.

DS20005173B-page 17

MCP16251/2

Another particular situation is powering a boost circuit

from one rechargeable cell like NiMH or NiCd. These

applications need an external MCU to monitor the cell

voltage or a separate UVLO circuit to prevent deep

discharging, which results in permanent cell damage.

In a multi-cell powered application (e.g., 2.4V typ. from

two NiMH cells) deep discharging will result in a

reverse polarity charging of one of the cells due the

unbalanced

cell

voltages,

thus

damaging

the

respective cell.

5.6.1

UVLO CIRCUIT FOR SINGLE-CELL

APPLICATIONS

In single-cell battery applications, it is not always

possible to monitor the start-up and shut-down

voltages using a MCU (due to MCU’s features or to I/O

pin count limitations).

One simple way to implement a ULVO circuit is a

simple diode placed between the VIN pin (with anode)

and EN pin (with cathode) and a bias resistor from EN

to GND.

Another example which shows an additional ultra low

power undervoltage lockout (UVLO) circuit with Start

and Stop thresholds is shown in Figure 5-3. Both

thresholds are programmable (1.15V to start and 0.8V

to stop the converter functionality) and new values can

be easily calculated using Equations 5-3 and 5-4. The

UVLO circuit consumes 0.5 µA up to max. 2 µA from

one battery.

FIGURE 5-3:

UVLO Circuit Example of a

Low Voltage Boost Converter with 1.15V Start

and 0.8V Stop Thresholds.

EQUATION 5-3:

EQUATION 5-4:

5.7

Thermal Calculations

The MCP16251/2 is available in two different packages

(SOT-23-6 and 2x3x0.8 TDFN-8). 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 MCP16251/2 family of devices is +125°C.

To quickly estimate the internal power dissipation for

the switching boost regulator, an empirical calculation

using measured efficiency can be used. Given the

measured efficiency, the internal power dissipation is

estimated by Equation 5-5.

EQUATION 5-5:

The difference between the first term, input power, and

the second term, power delivered, is the internal

MCP16251/2 power dissipation. This is an estimate

which assumes that most of the power lost is internal to

the MCP16251/2 and not to the CIN, COUT and/or the

inductor. There is some percentage of power lost in the

boost inductor, with very little loss in the input and

output capacitors. For a more accurate estimation of

internal power dissipation, subtract the IINRMS

2 x L

ESR

power dissipation.

5.8

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 MCP16251/2 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.

2M



2.2 M



R2

R3

2.4 M



R1

Q2

MMBT2222

3M



R4

to VIN

+

-

to EN

1M



R5

MMBT2907

Q1

ULVO

START

0.4

R

1

R

2

R

3

++



R

1

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





ULVO

STOP

0.4

R

1

R

2

+



R

1

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





where:

0.4 is VEBQ1 for selected M resistors range.

It increases to 0.6 if resistors are in k

 range.

V

OUT

I

OUT



Efficiency

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





V

OUT

I

OUT





–

P

Dis

=

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