B1121-Z-3.0 View Datasheet(PDF) - Bay Linear
Part Name
Description
MFG CO.
'B1121-Z-3.0' PDF : 9 Pages View PDF
APPLICATION HINTS
EXTERNAL CAPACITORS
The stability of the B1121 requires a 2.2µF or greater
capacitor between output and ground. Oscillation could
occur without this capacitor. Most types of tantalum or
aluminum electrolytic works fine here. For operations of
below -25°C solid tantalum is recommended since the
many aluminum types have electrolytes the freeze at about
-30°C. The ESR of about 5Ω or less and resonant
frequency above 500kHz are the most important
parameters in the value of the capacitor. The capacitor
value can be increased without limit.
At lower values of output current, less output capacitance
is required for stability. For the currents below 10mA the
value of the capacitor can be reduced to 0.5µF and 0.15µF
for 1mA. More output capacitance needed for the 8-pin
version at voltages below 5V since it runs the error
amplifier at lower gain. At worst case 5µF or greater must
be used for the condition of 250mA load at 1.23V output.
The B1121, unlike other low dropout regulators will
remain stable and in regulation with no load in addition to
the internal voltage divider. This feature is especially
important in application like CMOS RAM keep-alive.
When setting the output voltage of the B1121, a minimum
load of 10mA is recommended.
If there is more than 10 inches of wire between the input
and the AC filter capacitor or if a battery is used as the
input then a 0.1µF tantalum or aluminum electrolytic
capacitor should be placed from the input to the ground.
Instability can occur if there is stray capacitance to the
B1121 feedback terminal (pin 7). This could cause more
problems when using a higher value of external resistors to
set the output voltage
4.75V
OUTPUT
VOLTAGE
_______
ERROR*
+5.0V
+ INPUT
VOLTAGE 1.3V
+
+
* See Application Info. _______
Figure 1. ERROR Output Timing
B1121
This problem can be fixed by adding a 100pF capacitor
between output and feedback and increasing the output
capacitor to at least 3.3µF.
ERROR DETECTION COMPARATOR OUTPUT
The Comparator produces a logic low output whenever the
B1121 output falls out of regulation by more than around
5%. This is around 60mV offset divided by the 1.235
reference voltage. This trip level remains 5% below normal
regardless of the programmed output voltage of the
regulator. Figure 1 shows the timing diagram depicting the
ERROR signal and the regulator output voltage as the
B1121 input is ramped up and down. The ERROR signal
becomes low at around 1.3V input, and goes high around 5V
input (input voltage at which Vout = 4.75). Since the
B1121’s dropout voltage is load dependent, the input
voltage trip point (around 5V) will vary with the load
current. The output voltage trip point (approx. 4.75V) does
not vary with load.
The error comparator has an open-collector output, which
requires an external pull-up resistor. Depending on the
system requirements the resistor may be returned to 5V
output or other supply voltage. In determining the value of
this resistor, note that the output is rated to sink 400µA, this
value adds to battery drain in a low battery condition.
Suggested values range from 100K to 1MΩ. If the output is
unused this resistor is not required.
PROGRAMMING THE OUTPUT VOLTAGE OF
B1121 (SO-8 Version)
The B1121 may be pin-strapped for 5V using its internal
voltage divider by tying Pin 1 (output) to Pin 2 (sense) and
Pin 7 (feedback) to Pin 6 (5V Tap).
Also, it may be programmed for any output voltage between
its 1.235V reference and its 30V maximum rating. As seen
in Figure 2, an external pair of resistors is required.
Refer to the below equation for the programming of the
output voltage::
VOUT = VREF × ( 1 + R1\ R2 )+ IFBR1
The VREF is 1.235 and IFB is the feedback bias current,
nominally -20nA. The minimum recommended load current
of 1 µA forces an upper limit of 1.2 MΩ on value of R2. If
no load is presented the IFB produces an error of typically
2% in VOUT, which may be eliminated at room temperature
by trimming R1. To improve the accuracy choose the value
of R2 = 100k this reduces the error by 0.17% and increases
the resistor program current by 12µA. Since the B1121
typically draws 60 µA at no load with Pin 2 open-circuited
this is a small price to pay
Bay Linear, Inc 2478 Armstrong Street, Livermore, CA 94550 Tel: (925) 989-7144, Fax: (925) 940-9556
www.baylinear.com
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