MP28200 – 5.5V, 200

MP28200 Rev.1.0

www.MonolithicPower.com

12

10/10/2016

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OPERATION

The MP28200 has an ultra-low, quiescent

current, step-down converter and low-dropout

regulator. The step-down converter has 500nA

extremely high efficiency at an ultra-low load

current.

Constant-On-Time Control of the Buck

The MP28200 uses a constant-on-time control

scheme to implement output voltage regulation.

The one-shot on-timer is controlled by the input

and output voltage conditions. At different input

and output voltage conditions, the switching

frequency is fairly the same stable, which helps

with system design. The switching frequency is

around 1.5MHz, typically.

With constant-on-time control, the output ripple

is small, and the load transient response is fast.

Constant-on-time control minimizes the need

for input and output capacitors.

The

MP28200

enters

pulse-skip

mode

automatically when the low-side switch current

reaches the zero ampere threshold. Pulse-skip

mode helps improve light-load efficiency. The

constant-on-time scheme provides a seamless

transition from pulse-width modulation (PWM)

mode to pulse-frequency modulation (PFM)

mode and vice versa.

Light-Load Operation

When the load current decreases and the low-

side switch current reaches the zero ampere

threshold, both the high-side and low-side

switches are turned off. Output energy is

provided by the output capacitors during this

period until the output voltage drops, reaches

the regulation voltage, and triggers another on

pulse.

Generally, the switching frequency in PFM

mode depends on the load current. The

switching frequency is lower when the load

current is lighter. With PFM control at light-load

mode plus the ultra-low quiescent operation

current, the MP28200 can achieve the highest

efficiency at an extremely low load. This helps

extend the charge cycle of any battery-powered

system.

When the buck works in light-load operation, it

needs at least 5µs to exit light load. When a

large, quick, and sharp load increase occurs in

light-load mode, the output voltage drops during

the exit transition.

Control (CTRL)

CTRL1/2/3 are used to control start-up and set

the output voltages of the step-down regulator.

When CTRL1/2/3 are low, the step-down

switcher of the MP28200 is disabled. Once

either one of CTRL1/2/3 are pulled high, the

switcher is enabled. The output voltage is set

depending on which CTRL pin is pulled high.

The output voltage is programmable according

to Table 1.

Table 1: CTRL vs. Output Voltages

CTRL3

CTRL2

CTRL1

OUT

0

0

0

Disabled

0

0

1

0.8V

0

1

0

1.0V

0

1

1

1.2V

1

0

0

1.5V

1

0

1

1.8V

1

1

0

2.5V

1

1

1

3.3V

The output voltage can be programmable

during operation and supports dynamic output

voltage scaling. CTRL cannot be floating. Any

used CTRL voltage cannot be less than VIN,

and any unused CTRL pin must be tied to GND.

Soft Start (SS)

When the converter is enabled, the internal

reference is powered up. After a certain delay

time, the device enters soft start (SS). The step-

down switcher output voltage ramps up to the

regulation voltage in about 0.5ms.

Power Good (PG) Indicators of the Buck

The MP28200 has an open-drain output power

less than 400Ω. PG requires an external pull-up

resistor (100kΩ~500kΩ) for the power good

indicator. This resistor can be pulled up to VIN

or tied to CTRL if the CTRL voltages do not

need to be adjusted dynamically.