LTC3858 Linear Technology, LTC3858 Datasheet - Page 26

• Manufacturer Part Number: LTC3858
• Description: Synchronous Step-Down Controller
• Manufacturer: Linear Technology

LTC3858
A second, more severe transient is caused by switching
in loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
alter its delivery of current quickly enough to prevent this
sudden step change in output voltage if the load switch
resistance is low and it is driven quickly. If the ratio of
C
should be controlled so that the load rise time is limited
to approximately 25 • C
require a 250µs rise time, limiting the charging current
to about 200mA.
Design Example
As a design example for one channel, assume V
12V(nominal), V
V
The inductance value is chosen first based on a 30% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the FREQ pin
to GND, generating 350kHz operation. The minimum
inductance for 30% ripple current is:
A 4.7µH inductor will produce 29% ripple current. The
peak inductor current will be the maximum DC value plus
one half the ripple current, or 5.73A. Increasing the ripple
current will also help ensure that the minimum on-time
of 95ns is not violated. The minimum on-time occurs at
maximum V
The equivalent R
using the minimum value for the maximum current sense
threshold (64mV):

applicaTions inForMaTion
LOAD
SENSE(MAX)
R
∆I
t
ON MIN
SENSE
L
OUT
to C
(
=
( )( )
, causing a rapid drop in V
V
f L
)
OUT
≤
OUT
=
IN
= 75mV and f = 350kHz.
5 73
64
:
V
is greater than 1:50, the switch rise time
V
.
IN
OUT

 
mV
IN
SENSE
1–
( )
= 22V (max), V
A
f
=
V
=
resistor value can be calculated by
V
OUT
LOAD
0 011
IN
22 350
.
V

 
. Thus a 10µF capacitor would
(
3 3
Ω
.
V
kHz
OUT
OUT
)
= 3.3V, I
=
. No regulator can
429
ns
MAX
= 5A,
IN
=
Choosing 0.5% resistors: R
an output voltage of 3.296V.
The power dissipation on the topside MOSFET can be easily
estimated. Choosing a Fairchild FDS6982S dual MOSFET
results in: R
maximum input voltage with T(estimated) = 50°C:
A short-circuit to ground will result in a folded back cur-
rent of:
with a typical value of R
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
which is less than full-load conditions.
C
temperature assuming only this channel is on. C
chosen with an ESR of 0.02Ω for low output ripple volt-
age. The output ripple in continuous mode will be highest
at the maximum input voltage. The output voltage ripple
due to ESR is approximately:
V
IN
I
P
P
is chosen for an RMS current rating of at least 3A at
SC
MAIN
SYNC
ORIPPLE
=
0 015
=
=
32
.
(
= R



(
DS(ON)
3 3
22
0 035
mV
2 98
5
.
.
.
V
V
Ω
V
ESR
– .
( )
1
A
–
= 0.035Ω/0.022Ω, C
5
2 3
Ω Ω
(∆I
) (
A
2
1
2
)



V
+
2
1 125 0 022
L
95


) = 0.02Ω(1.45A) = 29mV
(
+
DS(ON)
.
1
22
ns
4 7
+
2 3
A
.
.
V
= 24.9k and R
(
1
(
)(
0 005 50
µH
)
V
22
2
and δ = (0.005/°C)(25°C)
.

 

5
.
V
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2
(
A
350
)

 =

)
(
2 5
(
Ω
.
kHz
MILLER
2 98
)
Ω
°
.
=
C
B
)(
)
= 77.7k yields
220
–
=
215
A
25
= 215pF . At
331
mW
°
pF
C
mW
P-P
)
OUT
)


•
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is