Typical Performance Characteristics: continued
Adjust Pin Current vs Output Current
77.00
76.00
V
V
POWER=2.05
CONTROL=2.75V
75.00
74.00
73.00
72.00
0.00
1.00
2.00
3.00 4.00
5.00
6.00
7.00
8.00
Output Current (A)
Application Notes
Theory of Operation
Design Guidelines
The CS5257A-1 linear regulator provides adjustable volt-
ages from 1.25V to 5V at currents up to 7A. The regulator
is protected against short circuits, and includes a thermal
shutdown circuit with hysteresis. The output, which is cur-
rent limited, consists of a PNP-NPN transistor pair and
requires an output capacitor for stability. A detailed pro-
cedure for selecting this capacitor is included in the
Stability Considerations section.
Adjustable Operation
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 current
to flow to ground. This current creates a voltage across R2
that adds to the 1.253V across R1 and sets the overall out-
put 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 neces-
sary. The output voltage is set according to the formula:
VPOWER Function
The CS5257A-1 utilizes a two supply approach to maxi-
mize 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 voltage 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 voltage comes up. The output can never come up
unregulated.
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 composi-
tion for best tracking over temperature. The divider resis-
tors should be placed physically as close to the load as pos-
sible.
The CS5257A-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.
VOUT
VCONTROL
CS5257A-1
VPOWER
VSENSE
Adjust
R1
R2
Output Voltage Sensing
The CS5257A-1 five terminal linear regulator includes a
dedicated 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 regula-
tion. 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 ]
