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SC339 Datasheet(PDF) 9 Page - Semtech Corporation

Part # SC339
Description  Ultra Low Output Voltage Linear FET Controller
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC339 Datasheet(HTML) 9 Page - Semtech Corporation

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© 2006 Semtech Corp.
www.semtech.com
SC339
POWER MANAGEMENT
This is a very severe example, since the output
voltage is so low, therefore the allowable window
is ver y small. See Solution 2 for an alternate
circuit. For higher output voltages the components
required will be less stringent.
The input capacitance needs to be large enough to stop
the input supply from collapsing below -5% (i.e., the
design minimum) during output load steps. If the input
to the pass MOSFET is not local to the supply bulk
capacitance then additional bulk capacitance may be
required.
MOSFET selection: since the input voltage to the SC339
is 5V±5%, the minimum available gate drive is:
V
GS = (4.4 ― 1.1025) = 3.3V
So a MOSFET rated for V
GS = 2.7V will be required, with an
R
DS(ON)(MAX) (over-temperature) given by:
(VIN(MIN) ― VOUT )
(
1.14 ― 1.05)
R
DSON(MAX) = ——————
= —————— =
36m
Ω
I
OUT(MAX)
2.5
MOSFET SELECTION
SO-8 Footprint
Vgs
Rds-on
Imax
FDS6682
1.7V
9m
Ω
14A
IRF7456
1.5V
6.5m
Ω
16A
1206 Footprint
Vgs
Rds-on
Imax
Si5406DC
0.6V
20m
Ω
9.5A
NTHS5404
0.6V
25m
Ω
7.2A
SOT-23 Footprint
Vgs
Rds-on
Imax
IRLMS2002
1V
30m
Ω
5.2A
FDN337N
(when Input
is > 1.2V
for 1.05V
output)
0.7V
65m
Ω
2.2A
Solution 2: Using Passive Droop
Passive droop allows us to use almost the full output
tolerance window for transients, therefore making the
output capacitor selection simpler and less
expensive. The trade-offs are the cost of the droop
resistor versus the reduction in output capacitor cost,
and the reduction in headroom which impacts MOSFET
selection. The top of the feedback chain connects to the
input side of R
DROOP, and the output is set for 1.075V. Thus
at no load, V
OUT will be 1.075V (or 1.05V + 2.4%) and at IOUT
= 2.5A, V
OUT will be 1.025V (or 1.05V - 2.4%).
If 1% set resistors are used, the total DC error will be
±2.25% = 24mV. Thus, at no load, the minimum output
voltage will be given by:
V
OUT(MIN_NO_LOAD) = 1.075 ― 0.024 = 1.051V
This leaves 53.5mV for transient response, giving:
53.5mV = 21.4mΩ and,
R
ESR(MAX) = ————
2.5A
2.5 • 1 • 10¯6 = 47μF
C
BULK(MIN) = ———————
53.5 •10¯3
Instead of 2 x 100μF, 12m
Ω capacitors, we can use 1 x
47μF, 15m
Ω capacitor.
Applications Information (Cont.)
1.075V
1.2V +/-5% IN
1.05V @ 2.5A
C1
0.1uF
R1
11.0k
C3
100uF, 25mOhm POSCAP
4
5
6
R2
10.0k
RDROOP
20mOhm
Q1
IN
GND
EN
DRV
ADJ
PGD
1
3
2
U1
SC339


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