GM317
3-TERMINAL 1.5A
ADJUSTABLE REGULATOR
Application Information
Basic Circuit Operation
The GM317 is a 3- terminal floating regulator. In operation, the GM317 develops and maintains a nominal 1.25V
reference (VREF) between its output and adjustment terminals. This reference voltage is converted to a
programming current flow through R2 to ground.
The regulated output voltage is given by:
VOUT
VIN
GM317
VREF
R1
IPROG
R2
Adjust
IADJ
R2
R1
VOUT = VREF (1 +
) + IADJ R2
VREF = 1.25V typical
Basic Circuit Configuration
Since the current from the adjustment terminal (IADJ) represents an error term in the equation, the GM317was
designed to control IADJ to less than 100µA and keep it constant. To do this, all quiescent operating current is
returned to the output terminal. This imposes the requirement for a minimum load current. If the load current is
less than this minimum, the output voltage will rise.
Since the GM317 isafloatingregulator, itisonlythe voltage differential across the circuit which is important to
performance, and operation at high voltages with respect to ground is possible.
Load Regulation
The GM317 is capable of providing extremely good load regulation, but a few precautions are needed to obtain
maximum performance. For best performance, the programming resistor R1 should be connected as close to
the regulator as possible to minimize line drops which effectively appear in series with the reference, thereby
degrading regulation. The ground end of R2 can be returned near the load ground to provide re
sensing and improve load regulation.
mote ground
External Capacitors
A 0.1µF disc or 1.0µF tantalum input bypass capacitor CIN is recommended to reduce the sensitivity to input line
impedance. The adjustment terminal may be bypassed to ground to improve ripple rejection. This capacitor CADJ
prevents ripple from being amplified as the output voltage is increased. A 10µF capacitor should improve ripple
rejection about 15 dB at 120Hz in a 10V application.
Although the GM317 is stable with no output capacitance, like any feedback circuit, certain values of external
capacitance can cause excessive ringing. An output capacitance CO in the form of a 1.0µF tantalum or 25µF
aluminum electrolytic capacitor on the output swamps this effect and insures stability.
Protection Diodes
When external capacitors are used with any IC regulator, it is sometimes necessary to add protection diodes to
prevent the capacitors from discharging through low current points into the regulator.
The figure below shows the GM317 with the recommended protection diodes for output for output voltage in
excess of 25V or high capacitance values (CO > 25µF, CADJ > 10µF). Diode D1 prevents CO from discharging
thru the IC during an input short circuit. Diode D2 protects against capacitor CADJ discharging through the IC
during an output short circuit. The combination of diodes D1 and D2 prevents CADJ from discharging through the
IC during an input short circuit.
D1
1N4002
VOUT
VIN
GM317
R1
CO
D2
1N4002
Adjust
CIN
CADJ
R2
5
GAMMA [ GAMMA MICROELECTRONICS INC. ]
