ACT5880
Rev 2, 03-Sep-13
CONFIGURABLE STEP-UP DC/DC REGULATOR
sense resistor tap. The voltage on the sense
resistor is regulated at 200mV, the output current is
then derived from the resistance and this 200mV
volume. Figure 6 shows the internal block diagram
and the external circuit for constant current
operation.
General Description
The step-up DC/DC regulator in the ACT5880 is
factory-programmable for the options of operation in
voltage regulation mode, constant current mode
and negative output mode, which are options
against ordering.
The output current can be controlled by feeding a
voltage DIMMING into the resistive network
composed of RCS, RDM1 and RDM2. The
DIMMING voltage can be an analogue signal or a
logic signal, which implements analogue current
control or PWM on/off control to the current output.
When using a logic signal as DIMMING for turning
on and off the constant current output, the on and
off delay limits the maximum available frequency
used for dimming, which is typically less than
2.2kHz. Refer to the waveform captures in
TYPICAL PERFORMANCE CHARACTERISTIC
section for more details.
The step-up DC/DC has 80ns typical minimum on-
time and 40ns minimum off-time, with the flexible
factory programmable loop compensation and slope
compensation, could output in large dynamic range
from slightly high than input to 40V.
The output voltage in voltage regulation mode and
the over voltage protection threshold, the switch
operation to the system clock phase alignment, and
the turn-on delay time are configurable though the
I2C interface. The circuit is enabled and starts to
output after the specified turn-on delay when the
ON[ ] bit is set to 1. When the ON[ ] is cleared, the
step-up DC/DC circuit is shutdown.
Figure 5:
When the interrupt function is not masked and the
power ok condition is not met, the circuit asserts
interrupt. The power ok condition for the voltage
regulation mode is that the output voltage is within
7% of the normal output voltage. For the constant
current mode, the power ok condition is the over-
voltage does not occur. The interrupt function is
masked by setting the nFLTMSK[ ] bit. The over-
voltage detection monitors the voltage seen at
voltage feedback error amplifier input, with an
threshold about 30mV to the 1.2V reference
voltage.
The REG3 Circuit for Voltage Regulation Mode.
When the on-time or off-time of the switching is
close to the respective minimums, pulse-skipping
happens to stay in regulation, which is normal when
the input voltage and the output voltage are too
close or too far.
Figure 6:
The REG3 Circuit for Current Mode.
See the STEP-UP DC/DC VOLTAGE SETTING
table for VSET code to voltage cross. This code to
voltage cross applies to both the output voltage and
the over voltage threshold.
Voltage Regulation Option
In voltage regulation mode operation, the output is
sampled at the OUT3 pin with an internal divider
network. Figure 5 shows the internal block diagram
and the external circuit for the step-up DC/DC in
voltage regulation mode.
Current Regulation Option
In current regulation mode operation, the current is
sensed in the sinking path at the low end current
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