9 2004 Semtech Corp. www.semtech.com
SC338(A)
POWER MANAGEMENT
allowable voltage droop. Therefore with dI = 2.5A,
t = 1µs, and dV = 13.1mV:
F191
101.13
1015.2
C
3
6
)MIN(BULK
µ=
•
••
=
−
−
So if we use 1% V
OUT
set resistors we would select 2 x
>100µF, 12mΩ POSCAPs for output capacitance (which
assumes that local ceramic bypass capacitors will
absorb the balance of the (6 - 5.3)mΩ ESR
requirement - otherwise 10mΩ capacitors should be
used).
If we use 0.1% set resistors, then the total DC error
becomes ±2.85% = ±29.9mV, leaving ±2.15% = 22.6mV
for the ESR spike. In this case:
Ω== m0.9
A5.2
mV6.22
R
)MAX(ESR
and
F111
106.22
1015.2
C
3
6
)MIN(BULK
µ=
•
••
=
−
−
So for 0.1% resistors we could use 2 x 100µF, 18mΩ
POSCAPs for output capacitance, or 1 x >100µF, 10mΩ
POSCAP.
Obviously this is a very severe example, since the output
voltage is so low and therefore the allowable window is
very small. See solution 2 below for an alternate
solution. 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
SC338(A) is 5V±5%, the minimum available gate drive
is:
V3.3)1025.14.4(V
GS
=−=
So a MOSFET rated for V
GS
= 2.7V will be required, with
an R
DS(ON)(MAX)
(over temp.) given by:
Ω=
−
=
−
=
m36
5.2
)05.114.1(
I
)VV(
R
)MAX(OUT
OUT)MIN(IN
)MAX)ON(DS
Obviously, if a 12V rail is available to power the SC338(A),
the number of FET options increases dramatically.
Applications Infomation (Cont.)
Solution 2: using passive droop.
or similar
1.075V
1.2V +/-5% IN
1.05V @ 2.5A
C1
0.1uF
R1
11.0k
C3
100uF, 25mOhm POSCAP
U1
SC338(A)
1
2
3
4
5
6
7
8
9
10
DRV1
ADJ1
EN1
PGD1
GND PGD2
EN2
ADJ2
DRV2
IN
R2
10.0k
RDROOP
20mOhm
Q1
IRF7311
4
1
2
3
5
6
78
Passive droop allows us to use almost the full output
tolerance window for transients, hence making the
output capacitor selection simpler and (hopefully)
cheaper. 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 I
OUT
= 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
±3.75% = 39mV. Thus at no load, the minimum output
voltage will be given by:
V036.1039.0075.1V
)LOAD_NO_MIN(OUT
=−=
This leaves 38.5mV for transient response, giving:
Ω==
m4.15
A5.2
mV5.38
R
)MAX(ESR
and
F65
105.38
1015.2
C
3
6
)MIN(BULK
µ=
•
••
=
−
−
Instead of 2 x 100µF, 12mΩ capacitors, we can use 1 x
100µF, 15mΩ capacitor.
Layout Guidelines
The advantages of using the SC338(A) to drive external
MOSFETs are a) that the bandgap reference and control
circuitry are in a die that does not contain high power
dissipating devices and b) that the device itself does not
