5805RHRF Datasheet, PDF (3/11 Pages) M.S. Kennedy Corporation – RAD HARD ULTRA LOW DROPOUT ADJUSTABLE POSITIVE LINEAR REGULATOR

APPLICATION NOTES
PIN FUNCTIONS
VIN A,B - These pins provide the input power connection to the
MSK5805RH. This is the supply that will be regulated to the output.
Both pins must be connected for proper operation.
VBIAS - This pin provides power to all internal circuitry including bias,
start-up, thermal limit and overcurrent latch. VBIAS voltage range is 2.9V
to 7.5V. VBIAS should be kept greater than or equal to VIN.
GND1 - Internally connected to signal ground, these pins should be
connected externally by the user to the circuit ground and the GND2 pins.
LATCH - The MSK5805RH LATCH pin is used for both current limit and
thermal limit. A capacitor between the LATCH pin and ground sets a
time out delay in the event of an over current or short circuit condition.
The capacitor is charged to approximately 1.6V from a 7.2µA (nominal)
current source. Exceeding the thermal limit will charge the latch capac-
itor from a larger current source for a near instant shutdown. Once the
latch capacitor is charged the device latches off until the latch is reset.
Momentarily pull the LATCH pin low, toggle the shutdown pin high then
low or cycle the power to reset the latch. Toggling the shutdown pin or
cycling the bias power both disable the device during the reset operation
(see SHUTDOWN pin description). Pulling the LATCH pin low immedi-
ately enables the device for as long as the LATCH pin is held low plus
the time delay to re-charge the latch capacitor whether or not the fault
has been corrected. Disable the latch feature by tying the LATCH pin
low. With the LATCH pin held low the thermal limit feature is disabled
and the current limit feature will force the output voltage to droop but
remain active if excessive current is drawn.
SHUTDOWN - There are two functions to the SHUTDOWN pin. It may be
used to disable the output voltage or to reset the LATCH pin. To activate
the shutdown/reset functions the user must apply a voltage greater than
1.3V to the SHUTDOWN pin. The voltage applied to the SHUTDOWN
pin can be greater than the input voltage. The output voltage will turn
on when the SHUTDOWN pin is pulled below the threshold voltage.
If the SHUTDOWN pin is not used, it should be connected to ground.
FB - The FB pin is the inverting input of the internal error amplifier. The
non-inverting input is connected to an internal 1.265V reference. This
error amplifier controls the drive to the output transistor to force the FB
pin to 1.265V. An external resistor divider is connected to the output, FB pin
and ground to set the output voltage.
GND2 - Internally connected to power ground, these pins should be
connected externally by the user to the circuit ground and the GND1 pins.
VOUT A,B - These are the output pins for the device. Both pins must
be connected for proper operation.
POWER SUPPLY BYPASSING
To maximize transient response and minimize power supply transients
it is recommended that a 33µF minimum tantalum capacitor is connected
between VIN and ground. A 0.1µF ceramic capacitor should also be used
for high frequency bypassing.
START UP OPTIONS
The MSK5805RH starts up and begins regulating immediately when
VBIAS and VIN are applied simultaneously. Applying VBIAS before VIN
starts the MSK5805RH up in a disabled or latched state. When starting
in a latched state the device output can be enabled either by pulling the
latch pin low to drain the latch capacitor or pulsing the shutdown pin high.
The shutdown pulse duration is partially dependent upon the size of the
latch capacitor and should be characterized for each application; 30uS
is typically adequate for a 1uF latch capacitor at 25°C. A momentary high
pulse on the shutdown pin can be achieved using the RC circuit below
if VIN rises rapidly. The resistor and capacitor must be selected based
on the required pulse duration, the rise characteristic of VIN and the
shutdown pin threshold (see shutdown pin threshold and current curves).
The shutdown pin can be held high and pulled low after VIN comes up
or the latch pin held low and released after VIN comes up to ensure
automatic startup when applying VBIAS before VIN. Either of the basic
circuits below can be adapted to a variety of applications for automatic
start up when VBIAS rises before VIN.
IADJ1 AND IADJ2 - The IADJ pins provide a method to adjust the cur-
rent limit. The current limit of the MSK5805RH is sensitive to the input
voltage. For lower input voltages the current limit is reduced. For higher
input voltages current limit is increased. Place a short across IADJ1 and
IADJ2 for maximum current. Place a resisitor across IADJ1 and IADJ2
to decrease the current limit.
OUTPUT CAPACITOR SELECTION
Low ESR capacitance at the output is required to maintain regulation
and stability. A single 150µA (AVX PN TAZX157K010L) in parallel with
ceramic decoupling capacitance (0.01µF typical) ensure good stability
margins and transient performance for the broadest range of applica-
tions. Lower value output capacitors can also provide acceptable per-
formance in applications with defined operating ranges. For example,
a single 47µF (AVX PN TAZH476K010L) performs well in lower current
applications. Additional frequency response compensation can be im-
plemented with a simple RC network from pin 6 to the output or ground.
Reference the MSK5805RH Evaluation Card Application Note (AN033)
for more information.
OVERCURRENT LATCH-OFF/LATCH PIN CAPACITOR
SELECTION
As previously mentioned, the LATCH pin provides over current/output
short circuit protection with a timed latch-off circuit. Reference the LATCH
pin description note. The latch off time out is determined with an external
capacitor connected from the LATCH pin to ground. The time-out period
is equal to the time it takes to charge this external capacitor from 0V
to 1.6V. The latch charging current is provided by an internal current
source. This current is a function of bias voltage and temperature (see
latch charging current curve). For instance, at 25°C, the latch charging
current is 7.2µA at VBIAS=3V and 8µA at VBIAS=7V.
In the latch-off mode, some additional current will be drawn from the
bias supply. This additional latching current is also a function of bias
voltage and temperature (see typical performance curves).
The MSK5805RH current limit function is directly affected by the input
and output voltages. Custom current limit is available; contact the factory
for more information.
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8548-111 Rev. F 5/15