Datasheet

LTC3859
32
3859fa
APPLICATIONS INFORMATION
The R
SENSE
resistor value can be calculated by using the
minimum value for the maximum current sense threshold
(43mV):
R
SENSE
≤
43mV
6.88A
= 0.006Ω
Choosing 1% resistors: R
A
= 25k and R
B
= 80.6k yields
an output voltage of 3.33V.
The power dissipation on the top side MOSFET can be easily
estimated. Choosing a Fairchild FDS6982S dual MOSFET
results in: R
DS(ON)
= 0.035/0.022, C
MILLER
= 215pF. At
maximum input voltage with T(estimated) = 50°C:
P
MAIN
=
3.3V
22V
(6A)
2
1+(0.005)(50° C − 25° C)
{}
(0.035Ω)+(22V)
2
6
5A
2
(2.5Ω)(215pF) •
1
5V − 2.3V
+
1
2.3V
⎧
⎨
⎩
⎫
⎬
⎭
(350kHz) = 433mW
A short-circuit to ground will result in a folded back
current of:
I
SC
=
20 mV
0.006Ω
−
1
2
95ns(22V)
3.9µH
⎧
⎨
⎩
⎫
⎬
⎭
= 3.07A
with a typical value of R
DS(ON)
and z = (0.005/°C)(25°C)
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
P
SYNC
=(2.23A)
2
(1.125)(0.022Ω) = 233mW
which is less than under full-load conditions.
The input capacitor to the buck regulator C
IN
is chosen
for an RMS current rating of at least 3A at temperature
assuming only this channel is on. C
OUT
is chosen with
an ESR of 0.02 for low output ripple. The output ripple
in continuous mode will be highest at the maximum
input voltage. The output voltage ripple due to ESR is
approximately:
V
ORIPPLE
= R
ESR
(DI
L
) = 0.02(1.75A) = 35mV
P-P
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the IC. These items are also illustrated graphically in the
layout diagram of Figure 11. Figure 12 illustrates the current
waveforms present in the various branches of the 2-phase
synchronous buck regulators operating in the continuous
mode. Check the following in your layout:
1. Are the top N-channel MOSFETs MTOP1 and MTOP2
located within 1cm of each other with a common drain
connection at C
IN
? Do not attempt to split the input
decoupling for the two channels as it can cause a large
resonant loop.
2. Are the signal and power grounds kept separate?
The combined IC signal ground pin and the ground
return of C
INTVCC
must return to the combined C
OUT
(–) terminals. The path formed by the top N-channel
MOSFET, Schottky diode and the C
IN
capacitor should
have short leads and PC trace lengths. The output
capacitor (–) terminals should be connected as close
as possible to the (–) terminals of the input capacitor
by placing the capacitors next to each other and away
from the Schottky loop described above.
3. Do the LTC3859 V
FB
pins’ resistive dividers connect to
the (+) terminals of C
OUT
? The resistive divider must be
connected between the (+) terminal of C
OUT
and signal
ground. The feedback resistor connections should not
be along the high current input feeds from the input
capacitor(s).
4. Are the SENSE
–
and SENSE
+
leads routed together with
minimum PC trace spacing? The fi lter capacitor between
SENSE
+
and SENSE
–
should be as close as possible
to the IC. Ensure accurate current sensing with Kelvin
connections at the sense resistor.
5. Is the INTV
CC
decoupling capacitor connected close
to the IC, between the INTV
CC
and the power ground
pins? This capacitor carries the MOSFET drivers’ current
peaks. An additional 1µF ceramic capacitor placed
immediately next to the INTV
CC
and PGND pins can
help improve noise performance substantially.