ET- AMS1084M-XX
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To ensure good transient response with heavy load current changes capacitor values on the order of 100µF are
used in the output of many regulators. To further improve stability and transient response of these devices larger
values of output capacitor can be used.
PROTECTION DIODES
Unlike older regulators, the AMS1084M family does not need any protection diodes
between the adjustment pin and the output and from the output to the input to
prevent over-stressing the die.
AMS1084M
Internal resistors are limiting the internal current paths on the AMS1084M
adjustment pin, therefore even with capacitors on the adjustment pin no protection
diode is needed to ensure device safety under short-circuit conditions.
Diodes between the input and output are not usually needed.
Microsecond surge currents of 50A to 100A can be handled by the internal diode between the input and output pins of the
device. In normal operations it is difficult to get those values of surge currents even with the use of large output
capacitances. If high value output capacitors are used, such as 1000µF to 5000µF and the input pin is instantaneously
shorted to ground, damage can occur. A diode from output to input is recommended, when a crowbar circuit at the input of
the AMS1084M is used. Normal power supply cycling or even plugging and unplugging in the system will not generate
current large enough to do any damage.
The adjustment pin can be driven on a transient basis ±25V, with respect to the output without any device degradation. As
with any IC regulator, none the protection circuitry will be functional and the internal transistors will break down if the
maximum input to output voltage differential is exceeded.
RIPPLE REJECTION
The ripple rejection values are measured with the adjustment pin bypassed. The impedance of the adjust pin capacitor at the
ripple frequency should be less than the value of R1 (normally 100Ω to 120Ω) for a proper bypassing and ripple rejection
approaching the values shown. The size of the required adjust pin capacitor is a function of the input ripple frequency. If
R1=100Ω at 120Hz the adjust pin capacitor should be 25µF. At 10kHz only 0.22µF is needed.
The ripple rejection will be a function of output voltage, in circuits without an adjust pin bypass capacitor. The output ripple
will increase directly as a ratio of the output voltage to the reference voltage (VOUT / VREF).
OUTPUT VOLTAGE
The AMS1084M series develops a 1.25V reference voltage between the output and
AMS1084M
the adjust terminal. Placing a resistor between these two terminals causes a
constant current to flow through R1 and down through R2 to set the overall output
voltage.
This current is normally the specified minimum load current of 10mA. Because IADJ
is very small and constant it represents a small error and it can usually be ignored.
LOAD REGULATION
True remote load sensing it is not possible to provide, because the AMS1084M is a
three terminal device. The resistance of the wire connecting the regulator to the
load will limit the load regulation.
AMS1084M
The data sheet specification for load regulation is measured at the bottom of the
package. Negative side sensing is a true Kelvin connection, with the bottom of the
output divider returned to the negative side of the load.
The best load regulation is obtained when the top of the resistor divider R1 is
connected directly to the case not to the load. If R1 were connected to the load, the
effective resistance between the regulator and the load would be:
Connected as shown Fig.3, RΡ is not multiplied by the divider ratio. Using 16-gauge wire the parasitic line resistance is about
0.004Ω per foot, translating to 4mV/ft at 1A load current. It is important to keep the positive lead between regulator and load
as short as possible and use large wire or PC board traces.
BEIJING ESTEK ELECTRONICS CO.,LTD
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ESTEK [ Estek Electronics Co. Ltd ]
