sc1470 Semtech Corporation, sc1470 Datasheet - Page 12

• Manufacturer Part Number: sc1470
• Description: Synchronous Buck Pseudo Fixed Frequency Power Supply Controller
• Manufacturer: Semtech Corporation

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:
For our example we will use R
R
Z
We will select a value of C
R
POWER MANAGEMENT
Z
C
V
V
RIPPLE_VBAT(MAX)
RIPPLE_VBAT(MIN)
FB
TOP
BAT
BOT
RIPPLE
RIPPLE
ESR
TOP
2005 Semtech Corp.
TOP
, and worst case no smaller than 10mV
is large enough. C
= 6.67k
(
= 14.3k , therefore:
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
and C
V
R
_
C
R
R
RIPPLE
VBAT
3
ESR
OUT
ESR
1
TOP
TOP
P-P
(
MIN
_
TOP
I
VBAT
TOP
I
RIPPLE
f
and V
= 60pF
)
RIPPLE
SW
TOP
F
TOP
can be calculated as follows,
(
should not be greater than
MIN
, is required in parallel with
TOP
required:
_
TOP
_
VBAT
)
RIPPLE_VBAT(MIN)
VBAT
, in order to ensure that
P-P
= 56pF. Calculating the
. 0
(
MIN
(
MAX
the above component
TOP
015
)
TOP
V
)
required to achieve
V
P
P
P
Ohms
= 20.0k
P
P-P
= 22mV
at minimum
P-P
P-P
and
. If
12
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.
For our example:
C
We will select 440µF, using two 220µF, 25m
capacitors in parallel. For smaller load release overshoot,
V
POSLIM
C
V
FB_VBAT(MIN)
OUT_ST_POS
FB
OUT(MIN)
OUT
OUT
_
VBAT
_
(
MIN
ST
(
MIN
_
)
TR
= 610µF.
)
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
_
OUT
OUT
VBAT
I
OUT
(
MIN
TOL
)
P-P
ERR
R
- good
I
TR
RIPPLE
BOT
TR
2
DC
V
R
V
_
V
VBAT
2
TOP
1
OUT
(
_
MAX
2
ST
R
_
)
POS
BOT
2
f
SW
1
TOP
2
_
www.semtech.com
SC1470
VBAT
F
:
OUT_ST_POS
(
MIN
)
C
TOP
, when
V
OUT
P
TR
P
,