SC338AIMSTRT Semtech, SC338AIMSTRT Datasheet - Page 9

• Manufacturer Part Number: SC338AIMSTRT
• Description: IC FET CONTROLLER DUAL 10-MSOP
• Manufacturer: Semtech
• Type: Positive Adjustabler

Specifications of SC338AIMSTRT

• Number Of Outputs: 2
• Voltage - Output: 0.5 ~ 1.8 V, 0.5 ~ 3.3 V
• Current - Supply: 500µA
• Voltage - Input: 5V, 12V
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: SC338AIMSTR

allowable voltage droop. Therefore with dI = 2.5A,
t = 1µs, and dV = 13.1mV:
So if we use 1% V
>100µF, 12m
assumes that local ceramic bypass capacitors will
absorb the balance of the (6 - 5.3)m
requirement - otherwise 10m
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:
So for 0.1% resistors we could use 2 x 100µF, 18m
POSCAPs for output capacitance, or 1 x >100µF, 10m
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:
So a MOSFET rated for V
an R
Obviously, if a 12V rail is available to power the SC338(A),
the number of FET options increases dramatically.
R
R
POWER MANAGEMENT
Applications Infomation (Cont.)
C
C
V
GS
ESR
DS
2004 Semtech Corp.
BULK
BULK
(
ON
DS(ON)(MAX)
(
MAX
(
(
MIN
MIN
4 (
)
MAX
4 .
)
)
)
)
22
2
2
. 1
(over temp.) given by:
22
13
2
5 .
5 .
(
6 .
1025
V
5 .
1 .
6 .
POSCAPs for output capacitance (which
IN
mV
A
I
1
1
(
MIN
OUT
OUT
10
10
)
10
10
)
(
MAX
set resistors we would select 2 x
3
9
3
3
6
6
V
0 .
3 .
GS
)
OUT
m
V
191
111
= 2.7V will be required, with
)
and
F
F
(
. 1
capacitors should be
14
2
5 .
. 1
05
)
36
m
ESR
9
Solution 2: using passive droop.
1.05V @ 2.5A
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
Thus at no load, V
and at I
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:
This leaves 38.5mV for transient response, giving:
Instead of 2 x 100µF, 12m
100µF, 15m
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
R
C
V
1.2V +/-5% IN
OUT
ESR
BULK
100uF, 25mOhm POSCAP
(
(
MIN
MAX
(
MIN
OUT
_
)
)
NO
= 2.5A, V
_
C3
38
2
LOAD
38
2
5 .
5 .
5 .
capacitor.
5 .
RDROOP
20mOhm
mV
DROOP
)
A
1
OUT
10
OUT
. 1
10
, and the output is set for 1.075V.
will be 1.075V (or 1.05V + 2.4%)
075
15
will be 1.025V (or 1.05V - 2.4%).
3
6
1.075V
R1
11.0k
R2
10.0k
4 .
C1
0.1uF
IRF7311
or similar
m
65
. 0
capacitors, we can use 1 x
039
Q1
F
1
and
. 1
SC338(A)
036
2
3
4
5
8
1
2
U1
www.semtech.com
DRV1
ADJ1
EN1
PGD1
GND
7
V
6
SC338(A)
DRV2
PGD2
ADJ2
5
EN2
IN
4
10
9
8
7
6
3