R1155 Datasheet, PDF (8/26 Pages) RICOH electronics devices division – 150 mA 24 V Input High Voltage Regulator

R1155x
NO.EA-270-150702
THEORY OF OPERATION
Power Activation
When starting up the IC using the input voltages of the VDD and CE pins simultaneously with no load, the both
pin voltages have to be 0.06 V/ ms or faster. When starting up the IC using the both pin voltages at 0.06 V/ ms
or slower with no load, the VDD pin has to be started up before the CE pin.
Thermal Shutdown Circuit
The R1155x contains a thermal shutdown circuit, which stops regulator operation if the junction temperature
of the R1155x becomes higher than 145°C (Typ.). Additionally, if the junction temperature after the regulator
being stopped decreases to a level below 120°C (Typ.), it restarts regulator operation. As a result the operation
of the thermal shutdown circuit causes the regulator repeatedly to turn off and on until the causes of
overheating are removed. As a consequence a pulse shaped output voltage occurs.
Reverse Current Protection Circuit
The R1155x includes a reverse current protection circuit, which stops the reverse current flowing from the
VOUT to VDD pins or to GND pin when VOUT becomes more than VIN.
Usually, the LDO using Pch output transistor contains a parasitic diode between VDD pin and VOUT pin.
Therefore, if VOUT is more than VIN, the parasitic diode becomes forward direction. As a result, the current flows
from VOUT pin to VDD pin.
The R1155x switches the mode to the reverse current protection mode before VIN becomes smaller than VOUT
by connecting the parasitic diode of Pch output transistor to the backward direction, and connecting the gate
to VOUT pin. As a result, the Pch output transistor is turned off and the all the current pathways from VOUT
pin to GND pin are shut down to maintain the reverse current lower than [IREV] of the Electrical Characteristics.
Switching to either the normal mode or to the reverse current protection mode is determined by the magnitude
of VIN voltage and VOUT voltage. For the stable operation, offset and hysteresis are set as the threshold. The
detection/ release thresholds of both normal and reverse current protection modes are specified by [VREV_DET]
and [VREV_REL] of the Electrical Characteristics. Therefore, the minimum dropout voltage under the small load
current condition is restricted by the value of [VREV_REL].
Figure 7 and Figure 8 show the normal operation mode and reverse current protection mode, respectively.
Figure 9 shows the detection/ release timing of reverse current protection function. When giving the VOUT pin
a constant-voltage and decreasing the VIN voltage, the dropout voltage will become lower than the [VREV_DET].
As a result, the reverse current protection starts to function to stop the load current. By increasing the dropout
voltage more than the [VREV_REL], the protection mode will be released to let the load current to flow. If the
dropout voltage to be used is smaller than [VREV_REL], the detection and the release may be repeated.
The operation coverage of the reverse current protection circuit is VOUT ≥ 1.5 V. However, under the condition
of VIN = 0 V, always the reverse current protection mode is operating.
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