MIC2111B Datasheet, PDF (14/35 Pages) Micrel Semiconductor – High-Performance, Multi-Mode, Step-Down Controller

Micrel, Inc.
Functional Description
The MIC2111B is a pin-programmable control-mode,
single-phase PWM buck controller. The control mode can
be programmed to either valley current mode or voltage
mode through a single pin. The device provides the
control and protection features necessary for driving
intelligent power stages in high-current, step-down,
DC/DC converters. The MIC2111B is also compatible
with DrMOS power stages and drivers that use current
sensing across the inductor. The MIC2111B provides a
single tri-state, PWM logic signal that works with either
power-stage modules or discrete-driver MOSFETs. It has
precision enable and power good (PG) functions for
sequencing of multiple power supplies and its frequency
can be programmed from 200kHz to 2MHz thereby
optimizing system size and system efficiency.
The device supports power-saving mode at light loads
when the MODE pin of the power stage is connected to
GND. Optional outside audio range operation is also
possible when the power stage is configured in light load
mode. The MIC2111B uses differential current sensing
for better current-limit accuracy. It also uses a dedicated
differential amplifier for remote output sensing to achieve
accurate output voltage control. The MIC2111B has a
high-gain transconductance amplifier for easier loop
compensation. External slope compensation can be
added through a resistor to avoid sub-harmonic
oscillations. The MIC2111B has programmable OCP,
output OVP and thermal OTP protections and offers
Micrel’s proprietary bi-directional, single-wire fault
communication for total system protection.
Control Architecture
The MIC2111B is a pin-programmable multi-mode,
single-phase PWM buck controller that can be operated
under valley-current-mode and voltage-mode control
architectures.
Valley Current Mode
When MIC2111B is programmed to a fixed-frequency,
valley current mode control architecture, the inductor
current is sensed by the voltage drop measured across
the DCR of the inductor (MIC2111B). The current is
sensed during the off period of the switching cycle and is
conditioned with the internal current sense amplifier. The
gain of the current sense amplifier is 30 V/V. The output
signal of the current sense amplifier is compared with the
current programmed by the error amplifier to determine
the correct duty cycle. Slope compensation is added via a
resistor between V_IN and the SLOPE pin. The MIC2111B
generates a (V_IN –V_OUT) proportional current and passes it
through a capacitor to generate the slope compensation
ramp.
MIC2111B
This slope compensation ramp is then added to comp
signal to avoid sub-harmonic oscillations for duty cycles
of less than 50%.
Calculation of R_SLOPE can be found in the Application
Information section.
Voltage Mode
The MIC2111B can also be configured as voltage-mode
control scheme for noise sensitive applications. Control-
loop compensation is external for providing maximum
flexibility in choosing the operating frequency and output
LC filter components. Ramp is generated by connecting a
resistor between VIN and SLOPE. An internal
transconductance error amplifier produces an integrated
error voltage at COMP that helps to provide higher DC
accuracy. The voltage at COMP sets the duty cycle using
a PWM comparator and a ramp generator. On the rising
edge of an internal clock, the PWM turns on. During this
ON time, inductor current ramps up. Internal comparator
turn OFF PWM once appropriate duty cycle is reached.
During this time inductor releases the stored energy as
the inductor current ramps down, providing current to the
output.
Oscillator Frequency
The MIC2111B has an internal oscillator wherein the
frequency can be set through an external resistor at the
FREQ pin. The switching frequency can be programmed
from 200kHz to 2MHz using Equation 1:
R_FREQ = 10¹¹/F_SW [Ω]
Eq. 1
Where:
F_SW = Desired switching frequency in Hz.
PWM Modes and Logic Levels
There are multiple versions of power stages currently on
the market that support different load currents. These
include DrMOS and other intelligent power-stages. All
these power stages contain a MOSFET driver, high-side
and low-side MOSFETs. These power stages require a
single tri-stated PWM control signal for control and
protection (Table 1).
October 13, 2015
14
Revision 2.1