HIGH RIPPLE-REJECTION AND LOW DROPOUT MIDDLE-OUTPUT CURRENT CMOS VOLTAGE REGULATOR
Rev.2.1_00
S-1133 Series
4. Thermal shutdown circuit
The S-1133 Series implements a thermal shutdown circuit to protect the device from damage due to overheating.
When the junction temperature rises to 150 °C (typ.), the thermal shutdown circuit operates and the regulator operation
stops. When the junction temperature drops to 120 °C (typ.), the thermal shutdown circuit is released and the
regulator operation resumes.
If the thermal shutdown circuit starts operating due to self-heating, the regulator operation stops and the output voltage
falls. When the regulator operation has stopped, no self-heat is generated and the temperature of the IC is lowered.
When the temperature has dropped, the thermal shutdown circuit is released, the regulator operation resumes, and
self-heat is generated again. By repeating this procedure, the output voltage waveform forms pulses. This
phenomenon, stopping and resuming the regulator operation, continues until the internal power consumption is
reduced by reducing either the input voltage or output current or both, or the ambient temperature is lowered.
Table 8
Thermal Shutdown Circuit
Operating : 150 °C (typ.)*1
Released : 120 °C (typ.)*1
*1. Junction temperature
VOUT Pin Voltage
VSS level
Set value
5. Externally setting output voltage
The S-1133 Series provides the types in which output voltage can be set via the external resistor (S-1133B00/S-
1133A00). With such types, the external voltage can be optionally set between 1.8 V and 8.2 V by connecting a
resistor (Ra) between the VOUT and VADJ pins and a resistor (Rb) between the VADJ and VSS pins.
The output voltage to be set is determined by the following formulas.
V
OUT = 1.8 + Ra × la ·························· (1)
By substituting Ia = IVADJ + 1.8/Rb to above formula (1),
OUT = 1.8 + Ra × (IVADJ + 1.8/Rb) = 1.8 × (1.0 + Ra/Rb) + Ra × IVADJ ··············· (2)
V
In above formula (2), Ra × IVADJ is a factor for the output voltage error.
Whether the output voltage error is minute is judged depending on the following (3) formula.
By substituting IVADJ = 1.8/RVADJ to Ra × IVADJ
VOUT = 1.8 × (1.0 + Ra/Rb) + 1.8 × Ra/RVADJ ··························(3)
If RVADJ is sufficiently larger than Ra, the error is judged as minute.
VOUT
VOUT
Ia
Ib
Ra
1.8 V
Rb
VADJ
RVADJ
VIN
IVADJ
VSS
Figure 21
The following expression is in order to determine output voltage VOUT = 3.0 V.
If resistance Rb = 2 KΩ, substitute internal resistance in adjust pin RVADJ = 200 kΩ (typ.) into (3),
Resistance Ra = (3.0/1.8-1) × ((2 k × 200 k)/(2 k + 200 k)) ≒ 1.3 kΩ
Caution The above connection diagrams and constants will not guarantee successful operation. Perform
thorough evaluation using the actual application to set the constants.
Seiko Instruments Inc.
19
SII [ SEIKO INSTRUMENTS INC ]
