sc1485itstr Semtech Corporation, sc1485itstr Datasheet - Page 13

• Manufacturer Part Number: sc1485itstr
• Description: Dual Synchronous Buck Pseudo Fixed Frequency Power Supply Controller
• Manufacturer: Semtech Corporation

We will select a value of 12.5m
design, which would be achieved by using two 25m
output capacitors in parallel. Note that for constant-on
converters there is a minimum ESR requirement for
stability which can be calculated as follows:
This criteria should be checked once the output
capacitance has been determined.
Now that we know the output ESR we can calculate the
output ripple voltage:
and
For our example:
V
Note that in order for the device to regulate in a
controlled manner, the ripple content at the feedback
pin, V
V
V
values should be revisited in order to improve this. Quite
often a small capacitor, C
the top feedback resistor, R
V
100pF. The value of C
where R
calculating the value of Z
Secondly calculating the value of C
this:
R
POWER MANAGEMENT
Z
C
V
V
RIPPLE_VBAT(MAX)
RIPPLE_VBAT(MIN)
FB
BAT
RIPPLE
RIPPLE
ESR
TOP
2005 Semtech Corp.
TOP
, and worst case no smaller than 10mV
is large enough. C
(
MIN
FB
_
_
, should be approximately 15mV
. 0
)
VBAT
VBAT
2
R
BOT
015
BOT
Z
2
(
(
MAX
MIN
TOP
1
is the bottom feedback resistor. Firstly
is less than 15mV
= 27mV
)
f
)
SW
V
R
_
C
R
R
RIPPLE
VBAT
3
ESR
OUT
ESR
1
TOP
P-P
(
MIN
_
TOP
I
VBAT
TOP
I
RIPPLE
f
and V
)
RIPPLE
SW
TOP
F
TOP
can be calculated as follows,
(
should not be greater than
MIN
, is required in parallel with
required:
_
TOP
_
VBAT
)
RIPPLE_VBAT(MIN)
VBAT
, in order to ensure that
P-P
. 0
(
MIN
(
MAX
the above component
TOP
015
)
V
)
required to achieve
V
maximum for our
P
P
P
Ohms
P
P-P
= 22mV
at minimum
P-P
P-P
. If
13
For our example we will use R
R
Z
We will select a value of C
value of V
For our example:
V
Next we need to calculate the minimum output
capacitance required to ensure that the output voltage
does not exceed the transient maximum limit, POSLIM
starting from the actual static maximum, V
a load release occurs:
For our example:
V
Where TOL
POSLIM
The minimum output capacitance is calculated as
follows:
This calculation assumes the absolute worst case
condition of a full-load to no load step transient occurring
when the inductor current is at its highest. The
capacitance required for smaller transient steps my be
calculated by substituting the desired current for the I
term.
V
POSLIM
C
V
TOP
FB_VBAT(MIN)
OUT_ST_POS
FB
BOT
OUT
OUT
_
VBAT
= 6.67k
= 14.3k , therefore:
_
(
MIN
ST
(
MIN
_
)
TR
)
POS
TR
= 1.224V
FB
= 1.296V
V
= 14.8mV
L
TR
RIPPLE
based upon the selected C
is the transient tolerance. For our example:
V
V
POSLIM
_
and C
OUT
OUT
VBAT
I
OUT
(
MIN
TOL
)
P-P
ERR
TOP
R
- good
I
TR
RIPPLE
= 60pF
BOT
TR
2
DC
V
TOP
R
V
_
V
VBAT
2
1
TOp
OUT
= 56pF. Calculating the
(
_
MAX
2
ST
R
_
TOP
)
POS
BOT
2
f
SW
1
= 20.0k
TOP
2
_
SC1485
www.semtech.com
VBAT
F
:
OUT_ST_POS
(
MIN
)
C
TOP
, when
and
V
OUT
P
TR
P
,