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LTC3853 Datasheet, PDF (30/36 Pages) Linear Technology – Triple Output, Multiphase Synchronous Step-Down Controller | |||
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LTC3853
APPLICATIONS INFORMATION
For channel 1, the DCRSENSE ï¬lter/divider values are:
R1= R1|| R2 = 1100Ω â
4.75k;
RD
0.23
R2 = R1⢠RD = 4.75k ⢠0.23 â
1430Ω
1â RD 1â 0.23
The power loss in R1 at the maximum input voltage is:
PLOSSR1
=
(VIN(MAX)
â VOUT
R1
)
â¢
VOUT
= (20V â 5V) ⢠5V = 15.8mW
4.75k
The respective values for Channel 2 are R1 = 4.75k, R2 =
1430Ω; and PLOSSR1 = 11.6mW. And for Channel 3 are
R1 = 2.21k, R2 = 1.82k; and PLOSSR1 = 10.2mW.
Burst Mode operation is chosen for high light load efï¬ciency
(Figure 14) by ï¬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 (9mΩ). At heavier loads, DCR sensing provides
higher efï¬ciency.
100
10
DCR
90
9mΩ
80
1
DCR
70
60
0.1
50
40
0.01
EFFICIENCY
POWER LOSS
0.01
0.1
1
10
LOAD CURRENT (mA)
3853 F14
Figure 14. Design Example Efï¬ciency vs Load
The power dissipation on the topside MOSFET can be easily
estimated. Choosing a Siliconix Si4816BDY dual MOSFET
results in: RDS(ON) = 0.023Ω/0.016Ω, CMILLER â
100pF.
At maximum input voltage with T(estimated) = 50°C:
PMAIN
=
5V
20V
(5)2 [1+
(0.005)(50°C
â
25°C)]
â¢
(0.023Ω)
+
(2
0V
)2
â
ââ
5A
2
â
â â
(2Ω)(1
00pF
)
â¢
â¡
â£â¢
5
1
â 2.3
+
1
2.3
â¤â¦â¥(500kHz)
=
243mW
A short-circuit to ground will result in a folded back cur-
rent of:
ISC
=
(1/ 3)75mV
0.009Ω
â
1â
2 ââ
90ns(20V)â
3.3μH â â
=
2.5A
with a typical value of RDS(ON) and δ = (0.005/°C)(25)
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
PSYNC
=
20V â 5V
20V
(2.
5A
)2
(1.
125)(0.0
16Ω)
= 84mW
which is less than under full-load conditions.
CIN is chosen for an RMS current rating of at least 2A at
temperature assuming only one channel is on. COUT is
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:
VORIPPLE = RESR (ÎIL) = 0.02Ω(1.5A) = 30mVP-P
3853f
30
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