Datasheet
LT1790
23
1790fb
APPLICATIONS INFORMATION
More Output Current
The circuit in Figure 16 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 V
S
. 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
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 fl at 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.
–
+
LT1790-2.5
LT1782
17909 F16
C2
1μF
NOTE: NOT CURRENT LIMITED
V
OUT
= 2.5V
I
LOAD
= 0mA to 300mA
R3
22Ω
5%
R1
680Ω
5%
V
S
2.8V TO 3.3V
NO LOAD
SUPPLY CURRENT
120μA
R2
100k
5%
C1
0.1μF
R4
1k
5%
VISHAY SILICONIX
Si3445DV
Figure 16. Compact, High Output Current, Low Dropout, Precision 2.5V Supply