MIC23099 Datasheet, PDF (16/26 Pages) Micrel Semiconductor – Single AA/AAA Cell Step-Up/Step-Down Regulators

Micrel, Inc.
Application Information
Overview
The MIC23099 is a dual output voltage, power-
management IC (PMIC) that has excellent light load
efficiency that operates from a single cell battery. The
PMIC has a synchronous boost regulator, a synchronous
buck regulator, inrush current limiting, fault detection, a
low battery monitor and warning circuitry. The
synchronous boost output voltage (V_OUT1) is enabled first
and is powered from the battery. Next the synchronous
buck output (V_OUT2), which is powered from the boost
output voltage, is enabled. This configuration allows
V_OUT2 to be independent of battery voltage, thereby
allowing the buck output voltage to be higher or lower
than the battery voltage.
The boost regulator is a current-mode PWM design that
incorporates a high-efficiency PFM light-load mode, while
the buck operates in PFM mode with constant peak
current control. The boost employs adaptive pulse width
control that minimizes output ripple and avoids output
ripple chatter commonly found in conventional micro
power boost regulators. In addition, the MIC23099
incorporates a frequency control scheme that minimizes
switching noise in the audio band.
The MIC23099 has an integrated low-battery monitor
function. The low-battery level is indicated by an external
LED connected to the LED pin. The LED is on when the
battery voltage is above the 1.2V threshold and flashes
when the battery voltage falls below the threshold. In
addition, a supervisor circuit monitors each output and
asserts a power good signal when the sequencing is
done or the power good output is pulled low when a fault
condition occurs.
Boost Regulator
The high-efficiency, micro-power synchronous boost
regulator operates from one alkaline or NiMH battery. It
offers true output disconnect to achieve a shutdown
quiescent current of less than 1.0µA, extending battery
life.
The boost regulator achieves high efficiency over a wide
output current range by operating in either PWM or PFM
mode. PFM mode provides the best efficiency at light
loads and PWM mode at heavy loads. Operating mode is
automatically selected according to output load
conditions. In PWM mode, the switching frequency is
1.0MHz, minimizing the solution foot-print.
The current-mode PWM design is internally
compensated, simplifying the design. Current mode
provides excellent line and load regulation as well as
cycle-by-cycle current limiting.
MIC23099
Also, an inrush current limiting feature is provided to
reduce the inrush current which minimizes the voltage
droop on the battery when the device is turned on.
Buck Regulator
The buck converter is designed to operate in PFM mode
with constant peak current control. When the buck
regulator high-side switch turns on, the inductor current
starts to rise. When the inductor current hits the current
limit threshold, a RS flip-flop is reset, turning off high-side
switch and on the low-side synchronous switch. The low-
side switch will remain on until the inductor current falls to
zero at which time it is turned off. Both switches will
remain off until the cycle repeats itself when the buck
feedback voltage falls below the internal 0.6V reference
and the internal comparator sets the RS flip-flop Q output
high.
Low-Battery Voltage Monitoring
The internal low input voltage monitor determines when
the input voltage is below the internally set 1.2V (typical)
threshold. When the input voltage falls below the
internally set threshold, the external LED connected to
the LED pin begins to blink at a frequency of 0.25Hz with
a duty cycle of 25%. The low input voltage threshold of
1.2V has a ±50mV variation.
Anti-Ringing Control
Both the buck and boost converters have an anti-ringing
control circuit that minimizes the ringing on the switching
node caused by the inductor and the parasitic
capacitance of the switch node when the synchronous
MOSFET turns off. When the inductor current falls to zero
an internal anti-ringing switch is connected across the
inductor. This temporally shorts the inductor and
eliminates the ringing on the switch node.
True Micro-Power Shutdown
This shutdown feature disconnects the boost output from
the battery. This feature eliminates power draw from the
battery through the synchronous switch during shutdown.
In conventional boost regulators, there is a catch diode
that provides a current path from the battery through the
inductor to the output of the boost regulator that can draw
current even when the regulator is shutdown.
May 27, 2014
16
Revision 1.2