ltc3850gn-2 Linear Technology Corporation, ltc3850gn-2 Datasheet - Page 29

• Manufacturer Part Number: ltc3850gn-2
• Description: Dual, 2-phase Synchronous Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
The power loss in R1 at the maximum input voltage is:
The respective values for Channel 2 are R1 = 4.12k, R2 =
1.5k; and P
Burst Mode operation is chosen for high light load effi ciency
(Figure 15) by fl oating the MODE/PLLIN pin. Power loss
due to the DCR sensing network is slightly higher at light
loads than would have been the case with a suitable sense
resistor (7mΩ). At heavier loads, DCR sensing provides
higher effi ciency.
P
(20V − 3.3V) • 3.3V
LOSS
Figure 15. Design Example Effi ciency vs Load
R1=
100
6.19k
90
80
70
60
50
40
LOSS
0.01
(V
IN(MAX)
R1 = 8mW.
LOAD CURRENT (mA)
0.1
= 9mW
− V
R1
OUT
) • V
1
EFFICIENCY
POWER LOSS
7mΩ
OUT
DCR
=
DCR
38502 F15
10
10
1
0.1
0.01
The power dissipation on the topside MOSFET can be easily
estimated. Choosing a Siliconix Si4816BDY dual MOSFET
results in: R
At 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
= 0.1. The resulting power dissipated in the bottom
MOSFET is:
which is less than under full-load conditions.
C
temperature assuming only channel 1 or 2 is on. C
chosen with an ESR of 0.02Ω 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:
IN
V
I
P
(
P
⎡
⎢
⎣
SC
0.023Ω
is chosen for an RMS current rating of at least 2A at
ORIPPLE
MAIN
SYNC
5 – 2.3
=
1
(
=
1/ 3
=
= 66mW
0.007Ω
3.3V
20V
20V – 3.3V
+
)
= R
DS(ON)
+ 20V
)
2.3
50mV
1
(
20V
ESR
( )
⎤
⎥ 500kHz
⎦
5
(
= 0.023Ω/0.016Ω, C
(ΔI
2
)
–
[
2
1+(0.005)(50°C – 25°C)
L
⎛
⎝ ⎜
2
1
(
) = 0.02Ω(1.5A) = 30mV
5A
2.1A
⎛
⎝ ⎜
2
DS(ON)
90ns(20V)
)
⎞
⎠ ⎟
= 186mW
3.3µH
( )
)
2Ω
2
(
and δ = (0.005/°C)(20)
1.125
(
LTC3850-2
100pF
⎞
⎠ ⎟
)
(
= 2.1A
MILLER
0.016Ω
)
•
]
≅ 100pF .
•
P–P
)
29
OUT
38502f
is