LT1790BCS6-3.3 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1790BCS6-3.3' PDF : 24 Pages View PDF
LT1790
APPLICATIONS INFORMATION
8
7
0°C TO 25°C
6
5
70°C TO 25°C
4
3
2
1
0
–80 –70 –60 –50 –40 –30 –20 –10 0 10 20 30 40 50
DISTRIBUTION (ppm)
1790 F08
Figure 8. Worst-Case 0°C to 70°C Hysteresis on 30 Units
4V TO 30V
R1
BZX84C18
C1
0.1µF
LT1790-3.3
1µF
VOUT
1790 F10
Figure 10. Extended Supply Range Reference
With R1 = 1k, and assuming a 450mV worst-case dropout,
the LT1790 can deliver a minimum current of (4V – 3.3V–
450mV)/(1k) = 250µA. In Figure 10, R1 and C1 provide
filtering of the zener noise when the zener is in its noisy V-I
knee.
There are other variations for higher voltage operation that
use a pass transistor shown in Figures 11 and 12. These
circuits allow the input voltage to be as high as 160V while
maintaining low supply current.
More Output Current
The circuit in Figure 13 is a compact, high output current,
low dropout precision supply. The circuit uses the SOT-23
LT1782 and the ThinSOT LT1790. Resistive divider R1 and
R2 set a voltage 22mV below VS. For under 1mA of output
current, the LT1790 supplies the load. Above 1mA of load
current, the (+) input of the LT1782 is pulled below the
22mV divider reference and the output FET turns on to
supply the load current. Capacitor C1 stops oscillations in
16
14
12
10
8
85°C TO 25°C
–40°C TO 25°C
6
4
2
0
–240 –200 –160 –120 –80 –40
0
DISTRIBUTION (ppm)
40
80
1790 F09
Figure 9. Worst-Case –40°C to 85°C Hysteresis on 30 Units
the transition region. The no load standing current is only
120µA, yet the output can deliver over 300mA.
Noise
An estimate of the total integrated noise from 10Hz to 1kHz
can be made by multiplying the flat band spot noise by
√BW. For example, from the Typical Performance Curves,
the LT1790-1.25 noise spectrum shows the average spot
noise to be about 450nV/√Hz. The square root of the
bandwidth is √990 = 31.4. The total noise 10Hz to 1kHz
noise is (450nV)(31.4) = 14.1µV. This agrees well with the
measured noise.
This estimate may not be as good with higher voltage
options, there are several reasons for this. Higher voltage
options have higher noise and they have higher variability
due to process variations. 10Hz to 1kHz noise may vary by
2dB on the LT1790-5 and 1dB on the LT1790-2.5.
Measured noise may also vary because of peaking in the
noise spectrum. This effect can be seen in the range of
1kHz to 10kHz with all voltage options sourcing different
load currents. From the Typical Performance Curves the
10Hz to 1kHz noise spectrum of the LT1790-5 is shown to
be 3µV/√Hz at low frequency. The estimated noise is
(3µV)(31.4) = 93.4µV. The actual integrated 10Hz to 1kHz
noise measures 118.3µV. The peaking shown causes this
larger number. Peaking is a function of output capacitor as
well as load current and process variations.
1790fa
20
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