MIC23099 Datasheet, PDF (17/26 Pages) Micrel Semiconductor – Single AA/AAA Cell Step-Up/Step-Down Regulators

Micrel, Inc.
Power-Up Sequencing
When the enable pin voltage rises above the enable
threshold voltage, the MIC23099 enters its start-up
sequence. Initially, the boost converter high-side PMOS
switch operates in linear mode and emulates a current
limited switch until the output voltage V_OUT1 reaches V_IN.
Then a fixed duty-cycle clock controls the boost converter
until V_OUT1 reaches 1.6V. When V_OUT1 is greater than 1.6V
the boost PFM control circuitry takes over until the output
reaches its regulated voltage value.
When V_OUT1 reaches 92.5% of its nominal value, V_OUT2 is
enabled. The power good output goes high 10ms to
50ms after V_OUT2 reaches the programmed value. Figure
1 waveforms detail the circuits operation.
MIC23099
The boost regulator operates in either PWM or PFM
mode. To avoid PWM to PFM chatter, the PWM entry
and exit points are not the same. When in PFM mode the
output current needs to reach 90mA to enter into PWM
mode and exits at 30mA. The boost switching frequency
is greater than 100kHz with loads greater than 20mW.
Figure 2. Boost Switching Frequency vs. Output Current
Figure 1. Power-Up Sequencing
Power Good
The power good (PG) circuitry monitors the battery
voltage and feedback pin voltage of the boost and buck
regulators. The PG pin output goes logic high when FB1
and FB2 pin voltages are both greater than 92.5%
(typical) of the internal reference voltage and the input
voltage is greater than 0.85V (typical). To minimize false
triggering, the power good output has both a turn on
delay and a falling deglitch delay.
Boost Switching Frequency
To reduce switching artifacts in the audio band, the buck
and boost regulators switching frequency are controlled
to minimize overlap. Figure 2 shows the boost switching
frequency versus output load current and Figure 3 shows
the buck. switching frequency versus output load current.
Buck Switching Frequency
The buck converter is designed to operate in PFM mode
only. It has peak current control, which turns off the high-
side switch when the inductor current hits the current limit
threshold. The cycle repeats itself when the output
voltage falls below its regulated value. As a result, the
switching frequency varies linearly with output current as
shown in Figure 3. The buck switching frequency is
greater than 80kHz with loads greater than 8mW.
May 27, 2014
Figure 3. Buck Switching Frequency vs. Output Current
17
Revision 1.2