Typical Performance Characteristics: continued
Minimum Load Current vs VPOWER-VOUT
916.400
916.300
916.200
916.100
916.000
915.900
915.800
VCONTROL = 5V
D VOUT = +1%
915.700
915.600
915.500
915.400
0.50
1.50
2.50
3.50
4.50
VPOWER-VOUT (V)
Application Notes
Design Guidelines
Theory of Operation
Adjustable Operation
The CS5258-1 linear regulator provides adjustable voltages
from 1.25V to 5V at currents up to 8A. The regulator is pro-
tected against short circuits, and includes a thermal shut-
down circuit with hysteresis. The output, which is current
limited, consists of a PNP-NPN transistor pair and requires
an output capacitor for stability. A detailed procedure for
selecting this capacitor is included in the Stability
Considerations section.
This LDO adjustable regulator has an output voltage range
of 1.25V to 5V. An external resistor divider sets the output
voltage as shown in Figure 1. The regulatorÕs voltage sens-
ing error amplifier maintains a fixed 1.253V 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 1.253V 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. The output voltage is set according to the
VPOWER Function
The CS5258-1 utilizes a two supply approach to maximize
efficiency. The collector of the power device is brought out
to the VPOWER pin to minimize internal power dissipation
under high current loads. VCONTROL provides power for
the control circuitry and the drive for the output NPN
transistor. VCONTROL should be at least 1V greater than the
output voltage. Special care has been taken to ensure that
there are no supply sequencing problems. The output volt-
age will not turn on until both supplies are operating. If
the control voltage comes up first, the output current will
be typically limited to about 3mA until the power input
voltage comes up. If the power input voltage comes up
first the output will not turn on at all until the control volt-
age comes up. The output can never come up unregulated.
formula:
R1+R2
VOUT = 1.253V ´
+ R2 ´ IADJ
(
)
R1
The term IADJ ´ R2 represents the error added by the adjust
pin current.
R1 is chosen so that the minimum load current is a least
10mA. R1 and R2 should be of the same composition for
best tracking over temperature. The divider resistors
should be located as close to the load as possible.
The CS5258-1 can also be used as a single supply device
with the control and power inputs tied together. In this
mode, the dropout will be determined by the minimum
control voltage.
VCONTROL VOUT
CS5258-1
VPOWER
VSENSE
Adjust
R1
R2
Output Voltage Sensing
The CS5258-1 five terminal linear regulator includes a ded-
icated VSENSE function. This allows for true Kelvin sensing
of the output voltage. This feature can virtually eliminate
errors in the output voltage due to load regulation.
Regulation will be optimized at the point where the sense
pin is tied to the output.
Figure 1: An external resistor divider sets the value of VOUT. The 1.253V
reference voltage drops across R1.
6
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
