MP28200 – 5.5V, 200mA, 1.5MHz, ULTRA-LOW I
Q
, STEP-DOWN CONVERTER
MP28200 Rev.1.0 www.MonolithicPower.com 12
10/10/2016 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2016 MPS. All Rights Reserved.
OPERATION
The MP28200 has an ultra-low, quiescent
current, step-down converter and low-dropout
regulator. The step-down converter has 500nA
of I
Q
current, allowing the MP28200 to achieve
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
good (PG) indicator with a maximum R
DS(ON)
of
less than 400. PG requires an external pull-up
resistor (100k~500k) 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.
