SI91821 Datasheet, PDF (6/10 Pages) Vishay Siliconix – Micropower 300-mA CMOS LDO Regulator With Error Flag

Si91821
Vishay Siliconix
DETAILED DESCRIPTION
The Si91821 is a low drop out, low quiescent current, and very
linear regulator with very fast transient response. It is primarily
designed for battery powered applications where battery run
time is at a premium. The low quiescent current allows
extended standby time while low drop out voltage enables the
system to fully utilize battery power before recharge. The
Si91821 is a very fast regulator with bandwidth exceeding
50 kHz while maintaining low quiescent current at light load
conditions. With this bandwidth, the Si91821 is the fastest
LDO available today. The Si91821 is stable with any output
capacitor type from 1 mF to 10.0 mF. However, X5R or X7R
ceramic capacitors are recommended for best output noise
and transient performance.
V_IN
V_IN is the input supply pin. The bypass capacitor for this pin
is not critical as long as the input supply has low enough source
impedance. For practical circuits, a 1.0-mF or larger ceramic
capacitor is recommended. When the source impedance is
not low enough and/or the source is several inches from the
Si91821, then a larger input bypass capacitor is needed. It is
required that the equivalent impedance (source impedance,
wire, and trace impedance in parallel with input bypass
capacitor impedance) must be smaller than the input
impedance of the Si91821 for stable operation. When the
source impedance, wire, and trace impedance are unknown,
it is recommended that an input bypass capacitor be used of
a value that is equal to or greater than the output capacitor.
V_OUT
V_OUT is the output voltage of the regulator. Connect a bypass
capacitor from V_OUT to ground. The output capacitor can be
any value from 1.0 mF to 10.0 mF. A ceramic capacitor with
X5R or X7R dielectric type is recommended for best output
noise, line transient, and load transient performance.
GND
Ground is the common ground connection for V_IN and V_OUT.
It is also the local ground connection for C_NOISE, SET, and SD.
SET
SET is not connected internally for the fixed voltage version.
Therefore, it can be connected to GND optionally. For the
adjustable output version, use a resistor divider R₁ and R₂,
connect R₁ from V_OUT to SET and R₂ from SET to ground. R₂
should be in the 25-kW to 150-kW range for low power
consumption, while maintaining adequate noise immunity.
The formula below calculates the value of R₁, given the
desired output voltage and the R₂ value.
ǒ Ǔ _{R1 +}
V_OUT * V_SET R₂
V_SET
(1)
V_SET is nominally 1.215 V.
SHUTDOWN (SD)
SD controls the turning on and off of the Si91821. V_OUT is
guaranteed to be on when the SD pin voltage equals or is
greater than 1.5 V. V_OUT is guaranteed to be off when the SD
pin voltage equals or is less than 0.4 V. During shutdown
mode, the Si91821 will draw less than 2-mA current from the
source. To automatically turn on V_OUT whenever the input is
applied, tie the SD pin to V_IN.
ERROR
ERROR is an open drain output that goes low when V_OUT is
less than 5% of its normal value. As with any open drain output,
an external pull up resistor is needed. This function is active
in shutdown.
The ERROR pin must be left opened if not used.
C_NOISE
For low noise application, connect a high frequency ceramic
capacitor from C_NOISE to ground. A 0.01-mF or a 0.1-mF X5R
or X7R is recommended.
www.vishay.com
6
Document Number: 71614
S-51147–Rev. E, 20-Jun-05