TM
MP2101 – 1.6MHz SYNCHRONOUS STEP-DOWN PLUS 200mA LDO
Short Circuit Protection
When the output is shorted to ground, the
oscillator frequency is reduced to prevent the
inductor current from increasing beyond the
PFET current limit, which is also reduced to
lower the short circuit current. The frequency
and current limit will return to the normal values
once the short circuit condition is removed and
the feedback voltage reaches 0.6V.
Maximum Load Current
The MP2101 can operate down to a 2.5V input
voltage. However the maximum load current
decreases at lower inputs due to a large IR
drop on the main switch and synchronous
rectifier. The slope compensation signal
reduces the peak inductor current as a function
of the duty cycle to prevent sub-harmonic
oscillations at duty cycles greater than 50%.
Conversely, the current limit increases as the
duty cycle decreases.
Power OK
The MP2101 provides an open-drain PWROK
output that goes high after both channels reach
regulation during startup. PWROK goes low
after one of the output channels goes out of
regulation by ±10% or when the device enters
shutdown. There is a built-in deglitch timer to
avoid a false PWROK trigger during load
transients (50µs for the switcher and 150µs for
the LDO). When the output is disabled, Power
OK remains low.
200mA Linear Regulator
The 200mA low dropout (LDO) linear regulator
has separate input (IN2) and output (OUT2)
pins for the internal power device. The control
circuitry of the LDO takes power from the main
input supply (IN1). Both IN1 and IN2 input
supplies must be present for the LDO to work
properly. The LDO power device input (IN2) can
be connected to the switcher output or directly
to the main supply (Figure 2). If IN2 is tied to
IN1, an optional RC filter can be inserted
between IN1 and IN2. The RC filter reduces
switching noise coupling from IN1 to IN2 and
power dissipation inside the MP2101. The
switcher guarantees 800mA output current, but
output current to the switcher load will be
reduced if the LDO draws current from the
switcher output.
800mA Step-Down Switcher
The switcher integrates both a main switch and
a synchronous rectifier, which provides high
efficiency and eliminates the need for an
external Schottky diode.
This switcher can achieve 100% duty cycle.
The duty cycle (D) of a step-down switcher is
defined as:
D
=
T
ON
×
f
OSC
×
100%
≈
V
OUT
×
100%
V
IN
Where T
ON
is the main switch on time and f
OSC
is the oscillator frequency (1.6MHz).
Current Mode PWM Control
Slope compensated current mode PWM control
provides stable switching and cycle-by-cycle
current limiting for superior load and line
response in addition to protection of the internal
main switch and synchronous rectifier. During
each cycle, the PWM comparator modulates
the power transferred to the load by changing
the inductor peak current based on the
feedback error voltage. During normal
operation, the main switch is turned on to ramp
the inductor current at each rising edge of the
internal oscillator, then switched off when the
peak inductor current is above the error voltage.
When the main switch is turned off, the
synchronous rectifier is immediately turned on
and stays on until the next cycle begins.
Dropout Operation
The MP2101 allows the main switch to remain
on for more than one switching cycle and
increases the duty cycle while the input voltage
is dropping close to the output voltage. When
the duty cycle reaches 100%, the main switch is
held on to deliver current to the output up to the
PFET current limit. The output voltage then
becomes the input voltage minus the voltage
drop across the main switch and the inductor.
MP2101 Rev. 1.0
8/18/2006
www.MonolithicPower.com
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