MIC23099 Datasheet, PDF (19/26 Pages) Micrel Semiconductor – Single AA/AAA Cell Step-Up/Step-Down Regulators

Micrel, Inc.
Boost Overcurrent Protection
The boost converter has current-limit protection on both
the high-side and low-side MOSFETs. The low-side
MOSFET provides cycle-by-cycle current limiting. When
the peak switch current exceeds the NMOS current limit
threshold, the low-side switch is immediately turned off
and the high-side switch is turned on. Peak switch current
is limited to approximately 1.5A. The low-side switch is
allowed to turn on again on the next clock cycle. If the
overload condition last more than 60ms to 120ms, both
outputs are disabled and the IC enters its power cycling
mode.
Component Selection
Resistors
An external resistive divider network (R1 and R2) with its
center tap connected to the feedback pin sets the output
voltage for each regulator. R1 is the top resistor and R2
is the bottom resistor in the divider string. The resistor
values for the desired output voltage are calculated as
illustrated in Equation 1. Large resistor values are
recommended to reduce light load operating current, and
improve efficiency. The recommended resistor value for
R1 should be around, R1 ≈ 400kΩ.
R2 = ^R1
 ^VOUT − 1
 ^0.6V 
Eq. 1
In the case of the boost converter, Equation 1 sets the
output voltage to its PWM value as shown in Figure 7.
The no-load PFM output voltage is 2% higher than the
PWM value. This higher PFM output voltage value is
necessary to prevent PFM to PWM mode skipping which
can introduce noise into the audio band.
Figure 7. Boost Load Regulation
Figure 8 shows the buck load regulation.
MIC23099
Figure 8. Buck Load Regulation
Inductor
Inductor selection is a balance between efficiency, cost,
size, switching frequency and rated current. For most
applications, inductors in the range 4.7µH to 6.8µH are
recommended. Larger inductance values reduce the
peak-to-peak ripple current, thereby reducing both the
DC losses and AC losses for better efficiency. The
inductor’s DC resistance (DCR) also plays an important
role. Since the majority of the input current (minus the
MIC23099 operating current) is passed through the
inductor, higher DCR inductors will reduce efficiency at
higher load currents.
The switch current limit for the MIC23099 is typically
1.5A. The saturation current rating of the selected
inductor should be 20 − 30% higher than the current limit
specification for the respective regulator.
Input Capacitor
The step-up converter exhibits a triangular, or sawtooth,
current waveform at its input, so an input capacitor is
required to decouple this waveform and thereby reduce
the input voltage ripple. A 4.7µF to 10µF ceramic
capacitor should be sufficient for most applications. A
minimum input capacitance of 1µF is recommended. The
input capacitor should be as close as possible to the
inductor, VIN pin, and PGND1 pin of the MIC23099.
Short, and wide, PCB traces are good for noise
performance.
Output Capacitor
Output capacitor selection is also a trade-off between
performance, size, and cost. Increasing the output
capacitor will lead to an improved transient response
performance. X5R and X7R ceramic capacitors are
recommended. For most applications, 10µF to 47µF
should be sufficient.
May 27, 2014
19
Revision 1.2