Applications Information
The CS5203-1 linear regulator provides adjustable volt-
ages in excess of 7V. The main considerations in such a
design are power-up and short circuit capability.
ages at currents up to 3A. The regulator is protected
against overcurrent conditions and includes thermal
shutdown.
In most applications, ramp-up of the power supply to VIN
is fairly slow, typically on the order of several tens of mil-
liseconds, while the regulator responds in less than one
microsecond. In this case, the linear regulator begins
charging the load as soon as the VIN to VOUT differential is
large enough that the pass transistor conducts current. The
load at this point is essentially at ground, and the supply
voltage is on the order of several hundred millivolts, with
the result that the pass transistor is in dropout. As the sup-
ply to VIN increases, the pass transistor will remain in
dropout, and current is passed to the load until VOUT
reaches the point at which the IC is in regulation. Further
increase in the supply voltage brings the pass transistor
out of dropout. The result is that the output voltage fol-
lows the power supply ramp-up, staying in dropout until
the regulation point is reached. In this manner, any output
voltage may be regulated. There is no theoretical limit to
the regulated voltage as long as the VIN to VOUT differen-
tial of 7V is not exceeded.
The CS5203-1 has a composite PNP-NPN output transistor
and requires an output capacitor for stability. A detailed
procedure for selecting this capacitor is included in the
Stability Considerations section.
Adjustable Operation
The CS5203-1 has an output voltage range of 1.25V to 5.5V.
An external resistor divider sets the output voltage as
shown in Figure 1. The regulator maintains a fixed 1.25V
(typical) reference between the output pin and the adjust
pin.
A resistor divider network R1 and R2 causes a fixed cur-
rent to flow to ground. This current creates a voltage
across R2 that adds to the 1.25V across R1 and sets the
overall output voltage. The adjust pin current (typically
50µA) also flows through R2 and adds a small error that
should be taken into account if precise adjustment of VOUT
is necessary.
However, the possibility of destroying the IC in a short
circuit condition is very real for this type of design. Short
circuit conditions will result in the immediate operation of
the pass transistor outside of its safe operating area. Over-
voltage stresses will then cause destruction of the pass
transistor before overcurrent or thermal shutdown circuit-
ry can become active. Additional circuitry may be required
to clamp the VIN to VOUT differential to less than 7V if fail-
safe operation is required. One possible clamp circuit is
illustrated in figure 2; however, the design of clamp cir-
cuitry must be done on an application by application basis.
Care must be taken to ensure the clamp actually protects
the design. Components used in the clamp design must be
able to withstand the short circuit condition indefinitely
while protecting the IC.
The output voltage is set according to the formula:
R1 + R2
VOUT = VREF
´
+ IAdj ´ R2
(
)
R1
The term IAdj ´ R2 represents the error added by the adjust
pin current.
R1 is chosen so that the minimum load current is at least
2mA. R1 and R2 should be the same type, e.g. metal film
for best tracking over temperature. While not required, a
bypass capacitor between the adjust pin and ground will
improve ripple rejection and transient response. A 0.1µF
tantalum capacitor is recommended for Òfirst cutÓ design.
Type and value may then be varied to optimize perfor-
mance vs. price.
EXTERNAL SUPPLY
VIN
VOUT
VIN
VOUT
VOUT
VIN
VAdj
C1
CS5203-1
VREF
R
C2
1
VOUT
Adj
IAdj
R
2
Figure 2. Short Circuit Protection Circuit for High Voltage Application.
Stability Considerations
CAdj
Figure 1. Resistor divider scheme.
The output or compensation capacitor helps determine
three main characteristics of a linear regulator: start-up
delay, load transient response and loop stability.
The 5203-1 linear regulator has an absolute maximum
specification of 7V for the voltage difference between VIN
and VOUT. However, the IC may be used to regulate volt-
The capacitor value and type is based on cost, availability,
size and temperature constraints. A tantalum or aluminum
5
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
