Definition of Terms
Dropout Voltage
Load Regulation
The change in output voltage for a change in load current
at constant chip temperature.
The input-output voltage differential at which the circuit
ceases to regulate against further reduction in input volt-
age. Measured when the output voltage has dropped
100mV from the nominal value obtained at 14V input,
dropout voltage is dependent upon load current and junc-
tion temperature.
Quiescent Current
The part of the positive input current that does not con-
tribute to the positive load current. The regulator ground
lead current.
Input Voltage
Ripple Rejection
The ratio of the peak-to-peak input ripple voltage to the
peak-to-peak output ripple voltage.
The DC voltage applied to the input terminals with respect
to ground.
Line Regulation
Current Limit
Peak current that can be delivered to the output.
The change in output voltage for a change in the input
voltage. The measurement is made under conditions of
low dissipation or by using pulse techniques such that the
average chip temperature is not significantly affected.
Circuit Description
circuitry insures that the output current never exceeds a
preset limit.
Voltage Reference and Output Circuitry
Precision Voltage Reference
The regulated output voltage depends on the precision
band gap voltage reference in the IC. By adding an error
amplifier into the feedback loop , the output voltage is main-
tained within ±4% over temperature and supply variation.
> 30V
V
IN
V
OUT
Output Stage
The composite PNP-NPN output structure (Figure1) pro-
vides 500mA (min) of output current while maintaining a
low drop out voltage (1.25V) and drawing little quiescent
current (7mA).
I
O
V
IN
Load
Dump
Short
Circuit
Thermal
Shutdown
Figure 2: Typical Circuit Waveforms for Output Stage Protection.
Should the junction temperature of the power device
exceed 180ûC (typ), the power transistor is turned off.
Thermal shutdown is an effective means to prevent die
overheating since the power transistor is the principle heat
source in the IC.
V
OUT
Figure 1: Composite Output Stage of the CS8140/1
Regulator Control Functions
The NPN pass device prevents deep saturation of the out-
put stage which in turn improves the ICÕs efficiency by
preventing excess current from being used and dissipated
by the IC.
The CS8140 differs from all other linear regulators in its
unique combination of control features.
Watchdog and ENABLE Functions
V
OUT is controlled by the logic functions ENABLE and
Output Stage Protection
Watchdog (Table 1).
The output stage is protected against overvoltage, short
circuit and thermal runaway conditions (Figure 2).
VOUT (V)
WDI
If the input voltage rises above 30V (e.g. load dump), the
output shuts down. This response protects the internal cir-
cuitry and enables the IC to survive unexpected voltage
transients.
ENABLE Slow
Normal
Fast
High
Low
H
L
5
0
5
5
5
0
5
0
5
0
Using an emitter sense scheme, the amount of current
through the NPN pass transistor is monitored. Feedback
Table 1: VOUT as a Function of ENABLE and Watchdog
6
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
