sc1470 Semtech Corporation, sc1470 Datasheet - Page 11

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

V
used to charge an internal 3.3pF capacitor to V
equations above reflect this along with any internal
components or delays that influence t
we select R
t
f
Now that we know t
for the inductor. To do this we select an acceptable
inductor ripple current. The calculations below assume
50% of I
and
For our example:
L
We will select an inductor value of 2.2µH to reduce the
ripple current, which can be calculated as follows:
and
For our example:
I
From this we can calculate the minimum inductor
current rating for normal operation:
L
L
I
I
I
POWER MANAGEMENT
RIPPLE_VBAT(MIN)
ON_VBAT(MIN)
SW_VBAT(MIN)
RIPPLE
RIPPLE
INDUCTOR
VBAT(MIN)
BAT
VBAT
VBAT
2005 Semtech Corp.
via R
(
(
_
_
MIN
MAX
VBAT
VBAT
(
)
= 1.3µH and L
MIN
OUT
)
tON
(
(
= 563ns and t
= 266kHz and f
MIN
MAX
)
, converting this to a current. This current is
which will give us a starting place.
tON
V
V
= 1.74A
)
BAT
)
I
BAT
OUT
= 1M :
(
MIN
(
V
MAX
(
V
MAX
BAT
BAT
)
)
)
ON
P-P
(
MIN
(
MAX
V
V
VBAT(MAX)
we can calculate suitable values
OUT
and I
I
)
RIPPLE
OUT
)
ON_VBAT(MAX)
V
SW_VBAT(MAX)
V
OUT
RIPPLE_VBAT(MAX)
_
t
OUT
VBAT
2
ON
= 1.6µH
0
t
ON
5 .
0
_
5 .
VBAT
_
(
MAX
t
VBAT
ON
t
= 255ns
I
ON
OUT
(
I
= 235kHz
)
_
MIN
OUT
ON
(
VBAT
_
A
MAX
L
VBAT
. For our example
)
(
L
MIN
H
= 2.18A
)
(
MIN
H
(
)
MAX
)
A
)
A
P
P
P
OUT
P-P
P
. The
11
For our example:
I
Next we will calculate the maximum output capacitor
equivalent series resistance (ESR). This is determined by
calculating the remaining static and transient tolerance
allowances. Then the maximum ESR is the smaller of the
calculated static ESR (R
(R
Where ERR
the DC error. The DC error will be 1% plus the
tolerance of the feedback resistors, thus 2% total for
1% feedback resistors.
For our example:
ERR
R
Where ERR
this calculation assumes that the worst case load
transient is full load. For half of full load, divide the I
term by 2.
For our example:
ERR
R
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:
R
R
INDUCTOR(MIN)
ESR_TR(MAX)
ESR_ST(MAX)
ESR
ESR
ESR_TR(MAX)
ST
TR
_
_
ST
TR
= 48mV and ERR
= 96mV and ERR
(
(
MAX
MAX
= 10.2m
= 22m
):
)
= 7.1A
)
TR
ST
is the transient output tolerance. Note that
is the static output tolerance and ERR
ERR
I
OUT
ERR
(MIN)
I
RIPPLE
ST
I
for a full 6A load transient
RIPPLE
TR
_
ERR
VBAT
DC
DC
ERR
ESR_ST(MAX)
= 24mV, therefore
_
= 24mV, therefore
(
VBAT
2
MAX
DC
)
(
DC
MAX
2
)
) and transient ESR
Ohms
Ohms
maximum for our
www.semtech.com
SC1470
DC
OUT
is