LT3028 LINER [Linear Technology], LT3028 Datasheet - Page 11

• Manufacturer Part Number: LT3028
• Description: Dual 100mA/500mA Low Dropout, Low Noise,Micropower Regulators with Independent Inputs
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
than 1µA in shutdown. In addition to the low quiescent
current, the LT3028 regulator incorporates several pro-
tection features which make it ideal for use in battery-
powered systems. The device is protected against reverse
input voltages. Additionally, in dual supply applications
where the regulator load is returned to a negative supply,
the output can be pulled below ground by as much as 20V
and still allow the device to start and operate.
Adjustable Operation
The LT3028 has an output voltage range of 1.22V to 20V.
The output voltage is set by the ratio of two external resis-
tors as shown in Figure 1. The device servos the output to
maintain the corresponding ADJ pin voltage at 1.22V ref-
erenced to ground. The current in R1 is then equal to
1.22V/R1 and the current in R2 is the current in R1 plus the
ADJ pin bias current. The ADJ pin bias current, 30nA at
25°C, flows through R2 into the ADJ pin. The output volt-
age can be calculated using the formula in Figure 1. The
value of R1 should be no greater than 250k to minimize
errors in the output voltage caused by the ADJ pin bias
current. Note that in shutdown the output is turned off and
the divider current will be zero. Curves of ADJ Pin Voltage
vs Temperature and ADJ Pin Bias Current vs Temperature
appear in the Typical Performance Characteristics.
The device is tested and specified with the ADJ pin tied to
the corresponding OUT pin for an output voltage of 1.22V.
Specifications for output voltages greater than 1.22V will
be proportional to the ratio of the desired output voltage
to 1.22V: V
Output 2 for an output current change of 1mA to 100mA
V
IN
IN1/IN2
OUT1/OUT2
LT3028
ADJ1/ADJ2
GND
OUT
Figure 1. Adjustable Operation
/1.22V. For example, load regulation on
U
R1
3024 F01
+
U
R2
V
OUT
I
OUTPUT RANGE
V
V
ADJ
OUT
ADJ
W
=
=
=
30
1 22
1 22
.
.
nA AT
V
V
⎛
⎜
⎝
1
25
+
= .
°
R
R
1 22
C
2
U
1
⎞
⎟ +
⎠
V TO
(
I
ADJ
20
)( )
R
V
2
is –1mV typical at V
regulation is:
Bypass Capacitance and Low Noise Performance
The LT3028 regulator may be used with the addition of a
bypass capacitor from V
to lower output voltage noise. A good quality low leakage
capacitor is recommended. This capacitor will bypass the
reference of the regulator, providing a low frequency noise
pole. The noise pole provided by this bypass capacitor will
lower the output voltage noise to as low as 20µV
the addition of a 0.01µF bypass capacitor. Using a bypass
capacitor has the added benefit of improving transient
response. With no bypass capacitor and a 10µF output
capacitor, a 10mA to 100mA load step on Output 2 will
settle to within 1% of its final value in less than 100µs. With
the addition of a 0.01µF bypass capacitor, the output will
stay within 1% for the same load step. Both outputs exhibit
this improvement in transient response (see Transient
Reponse in Typical Performance Characteristics section).
However, regulator start-up time is inversely proportional
to the size of the bypass capacitor, slowing to 15ms with
a 0.01µF bypass capacitor and 10µF output capacitor.
Output Capacitance and Transient Response
The LT3028 regulator is designed to be stable with a wide
range of output capacitors. The ESR of the output capaci-
tor affects stability, most notably with small capacitors.
A minimum output capacitor of 1µF with an ESR of 3Ω or
less is recommended for Output 2 to prevent oscillations.
A minimum output capacitor of 3.3µF with an ESR of 3Ω
or less is recommended for Output 1. The LT3028 is a
micropower device and output transient response will be
a function of output capacitance. Larger values of output
capacitance decrease the peak deviations and provide
improved transient response for larger load current
changes. Bypass capacitors, used to decouple individual
components powered by the LT3028, will increase the
effective output capacitor value. With larger capacitors
(12V/1.22V)(–1mV) = – 9.8mV
OUT
OUT
= 1.22V. At V
to the corresponding BYP pin
OUT
LT3028
= 12V, load
RMS
11
with
3028f