MCP1603LT-ADJISLASHOS datasheet(15/34 Pages) MICROCHIP

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 2007-2012 Microchip Technology Inc.

DS22042B-page 15

MCP1603/B/L

4.0

DETAILED DESCRIPTION

4.1

Device Overview

The MCP1603/L is a synchronous buck regulator that

operates in a Pulse Frequency Modulation (PFM)

mode or a Pulse Width Modulation (PWM) mode to

maximize system efficiency over the entire operating

current range. Capable of operating from a 2.7V to

5.5V input voltage source, the MCP1603 can deliver

500 mA of continuous output current.

The MCP1603B device disables the PFM mode

switching, and operates only in normal PWM mode.

When using the MCP1603/B/L, the PCB area required

for a complete step-down converter is minimized, since

both the main P-Channel MOSFET and the synchro-

nous N-Channel MOSFET are integrated. Also while in

PWM mode, the device switches at a constant

frequency of 2.0 MHz (typical), which allows for small

filtering components. Both fixed and adjustable output

voltage options are available. The fixed voltage options

(1.2V, 1.5V 1.8V, 2.5V, 3.3V) do not require an external

voltage divider, which further reduces the required

circuit board footprint. The adjustable output voltage

options allow for more flexibility in the design, but

require an external voltage divider.

Additionally, the device features an undervoltage lock-

out (UVLO), overtemperature shutdown, overcurrent

protection and enable/disable control.

4.2

Synchronous Buck Regulator

The MCP1603/L has two distinct modes of operation

that allow the device to maintain a high level of

efficiency throughout the entire operating current and

voltage range. The device automatically switches

between PWM mode and PFM mode, depending on

the output load requirements. MCP1603B switches in

PWM mode only.

4.2.1

PFM/PWM MODE DEVICE OPTION

(MCP1603/L)

During

heavy

load

conditions,

the

MCP1603/L

operates at a high, fixed switching frequency of

2.0 MHz (typical) using current mode control. This

minimizes output ripple (10 – 15 mV, typically) and

noise, while maintaining high efficiency (88% typical

with VIN =3.6V, VOUT =1.8V, IOUT = 300 mA).

During normal PWM operation, the beginning of a

switching cycle occurs when the internal P-Channel

MOSFET is turned on. The ramping inductor current is

sensed and tied to one input of the internal high-speed

comparator. The other input to the high-speed

comparator is the error amplifier output. This is the

difference between the internal 0.8V reference and the

divided-down output voltage. When the sensed current

becomes equal to the amplified error signal, the high-

speed comparator switches states and the P-Channel

MOSFET is turned off. The N-Channel MOSFET is

turned on until the internal oscillator sets an internal RS

latch, initiating the beginning of another switching

cycle.

PFM-to-PWM mode transition is initiated for any of the

following conditions:

• Continuous device switching

• Output voltage has dropped out of regulation

4.2.1.1

Light Load, PFM Mode

During light-load conditions, the MCP1603/L operates

in a PFM mode. When the MCP1603/L enters this

mode, it begins to skip pulses to minimize unnecessary

quiescent-current draw by reducing the number of

switching cycles per second. The typical quiescent cur-

rent draw for this device is 45 µA.

PWM-to-PFM mode transition is initiated for any of the

following conditions:

• Discontinuous inductor current is sensed for a set

duration

• Inductor peak current falls below the transition

threshold limit

4.2.2

PWM MODE DEVICE OPTION

(MCP1603B)

There are applications that cannot tolerate the low

frequency pulse skipping mode or the output ripple

voltage associated with it, which is distinctive for PFM

switching.

The MCP1603B device has disabled the PFM mode

switching. It operates only in normal PWM mode over

the entire load range (without skipping pulses). During

periods of light load operation, the MCP1603B

continues to operate at a constant 2 MHz switching

frequency, keeping the output ripple voltage lower than

PFM mode. Because there are no skipping pulses, a

minimum load current is necessary to keep output in

regulation (see Figure 2-15, without a minimum load,

the output voltage will be greater than the set point).

The minimum load value depends on the input-to-

output ratio.

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