SC338(A)
POWER MANAGEMENT
Applications Infomation (Cont.)
allowable voltage droop. Therefore with dI = 2.5A,
t = 1µs, and dV = 13.1mV:
Solution 2: using passive droop.
2.5
•
1
•
10
−
6
C
BULK (MIN)
=
=
191
µ
F
13.1
•
10
−
3
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:
1.2V +/-5% IN
C1
0.1uF
8
RDROOP
1.075V
C3
20mOhm
1
7
6
5
3
1.05V @ 2.5A
Q1
IRF7311
or similar
2
R1
1
2
3
R2
10.0k
4
5
U1
DRV1
ADJ1
EN1
PGD1
GND
4
SC338(A)
10
IN
9
DRV2
8
ADJ2
7
EN2
6
PGD2
100uF, 25mOhm POSCAP
11.0k
R
ESR(MAX )
22.6mV
=
=
9.0m
Ω
and
2 .5 A
2.5
•
1
•
10
−
6
C
BULK (MIN)
=
=
111
µ
F
−
3
22.6
•
10
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:
V
GS
=
( 4.4
−
1.1025 )
=
3.3 V
So a MOSFET rated for V
GS
= 2.7V will be required, with
an R
DS(ON)(MAX)
(over temp.) given by:
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:
V
OUT (MIN _ NO _ LOAD )
=
1.075
−
0.039
=
1.036 V
This leaves 38.5mV for transient response, giving:
R
ESR(MAX )
=
38.5mV
=
15.4m
Ω
and
2.5 A
2.5
•
1
•
10
−
6
C
BULK (MIN)
=
=
65
µ
F
38.5
•
10
−
3
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
9
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R
DS( ON)MAX )
=
( V
IN(MIN )
−
V
OUT
)
I
OUT (MAX )
=
(1.14
−
1.05 )
=
36m
Ω
2. 5
Obviously, if a 12V rail is available to power the SC338(A),
the number of FET options increases dramatically.
2004 Semtech Corp.
SEMTECH [ SEMTECH CORPORATION ]
