MCP1256 datasheet(12/24 Pages) MICROCHIP

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MCP1256/7/8/9

DS21989A-page 12

4.0

DEVICE OVERVIEW

The MCP1256/7/8/9 devices are positive regulated

charge pumps that accept an input voltage from +1.8V

to +3.6V and convert it to a regulated 3.3V output volt-

age. The MCP1256/7/8/9 provide a low-cost, compact

and simple solution for step-up DC/DC conversions,

primarily in battery applications, that do not want to use

switching regulator solutions because of EMI noise and

inductor size.

The MCP1256/7/8/9 are designed to offer the highest

possible efficiency under common operating condi-

tions,

i.e.

VIN = 2.4V

2.8V,

VOUT =3.3V,

IOUT = 100 mA. A fixed switching frequency, 650 kHz

typically, allows for easy external filtering.

The MCP1256/7 provide a unique SLEEP mode

feature which reduces the current drawn from the input

supply while maintaining a regulated bias on external

peripherals. SLEEP mode can substantially increase

battery run-time in portable applications.

The MCP1258/9 provide a unique BYPASS mode

feature which virtually eliminates the current drawn

from the input supply by the device while maintaining

an unregulated bias on external peripherals. BYPASS

connects the input supply voltage to the output. All

remaining functions of the device are shutdown.

BYPASS mode can substantially increase battery run-

time in portable applications.

The devices supply up to 100 mA of output current for

input voltages, VIN, greater than or equal to 2.2V. The

devices are available in small 10-Pin MSOP or DFN

packages with an operating junction temperature range

of -40°C to +125°C.

4.1

Theory of Operation

The MCP1256/7/8/9 devices employ a switched capac-

itor charge pump to boost an input supply, VIN, to a reg-

ulated 3.3V output voltage. Refering to the Functional

Block Diagram, the devices perform conversion and

regulation in two phases: charge and transfer. When

the devices are not in shutdown, SLEEP or BYPASS,

the two phases are continuously cycled through.

Charge transfers charge from the input supply to the

flying capacitors, C1 and C2, connected to pins C1+,

C1-, C2+ and C2-, respectively. During this phase,

switches S4 and S6 are closed. Switch S2 controls the

amount of charge transferred to the flying capacitors.

The amount of charge is determined by a sample and

hold error amplifier with feedback from the output

voltage at the beginning of the phase.

Once the first phase (charge) is complete, transfer is

initiated. The second phase transfers the energy from

the flying capacitors to the output. The MCP1256/7/8/9

devices autonomously switch between 1.5x mode and

2x mode. This determines whether the flying capacitors

are placed in parallel (1.5x mode), or remain in series

(2x mode), when the energy is transferred to the out-

put. The transfer mode determines which switches are

closed for the transfer.

Both phases occur in one clock period of the internal

oscillator. When the second phase (transfer) has been

completed, the cycle repeats.

4.2

Power Efficiency

The power efficiency,

η, is determined by the mode of

operation, 1.5x mode or 2x mode. Equation 4-1 and

Equation 4-2 are used to approximate the power effi-

ciency with any significant amount of output current. At

light loads, the device quiescent current must be taken

into consideration.

EQUATION 4-1:

EQUATION 4-2:

4.3

Shutdown Mode (SHDN)

Driving SHDN low places the MCP1256/7/8/9 in a low-

power Shutdown mode. This disables the charge-pump

switches, oscillator and control logic, reducing the

quiescent current to 0.25 μA (typical). The PGOOD

output and LBO are in a high impedance state during

shutdown.

4.4

SLEEP Mode (SLEEP)

The MCP1256/7 provide a unique SLEEP mode fea-

ture. SLEEP mode reduces the current drawn from the

input supply while maintaining a regulated bias on

external peripherals. SLEEP mode can substantially

increase battery run-time in portable applications.

The regulation control is referred to as a bang-bang

control due to the output being regulated around a fixed

reference with some hysteresis. As a result, some

amount of peak-to-peak ripple will be observed at the

output independent of load current. The frequency of

the output ripple, however, will be influenced heavily by

the load current and output capacitance.

4.5

BYPASS Mode (BYPASS)

The MCP1258/9 provide a unique BYPASS mode fea-

ture which virtually eliminates the current drawn from

the input supply by the device, while maintaining an

unregulated bias on external peripherals. BYPASS

connects the input supply voltage to the output. All

remaining functions of the device are shutdown.

BYPASS mode can substantially increase battery run-

time in portable applications.

1.5x

POUT

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

VOUT IOUT

VIN 1.5

IOUT

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

VOUT

VIN 1.5

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

POUT

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

VOUT IOUT

VIN 2

IOUT

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

VOUT

VIN 2

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

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