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LTC3865-1_15 Datasheet, PDF (32/38 Pages) Linear Technology – Dual, 2-Phase Synchronous DC/DC Controller with Pin Selectable Outputs | |||
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LTC3865/LTC3865-1
APPLICATIONS INFORMATION
With ILIM ï¬oating, the equivalent RSENSE resistor value
can be calculated by using the minimum value for the
maximum current sense threshold (44mV).
RSENSE(EQUIV)
=
VSENSE(MIN)
ILOAD(MAX)
+
ÎIL(NOM)
2
=
44mV
5A + 1.5A
â
7.7mΩ
2
The equivalent RSENSE is the same for Channel 2.
The Coiltronics (Cooper) HCP0703-2R2 (20mΩ DCRMAX
at 20°C) and HCP0703-3R3 (30mΩ DCRMAX at 20°C) are
chosen. At 100°C, the estimated maximum DCR values are
26.4mΩ and 39.6mΩ. The divider ratios are:
RD
=
RSENSE(EQUIV)
DCRMAX at TL(MAX)
=
7.7mΩ
26.4mΩ
= 0.3;
and 7.7mΩ â
0.2
39.6mΩ
For each channel, 0.1μF is selected for C1.
R1||R2 =
L
= 2.2μH
(DCRMAX at 20°C) ⢠C1 20mΩ ⢠0.1μF
= 1.1k ; and
3.3μH = 1.1k
30mΩ ⢠0.1μF
For channel 1, the DCRSENSE ï¬lter/divider values are:
R1= R1||R2 = 1.1k â
5.5k;
RD 0.2
R2 = R1⢠RD = 5.5k ⢠0.2 â
1.37k
1âRD 1â 0.2
The power loss in R1 at the maximum input voltage is:
PLOSSR1=
(VIN(MAX)
â VOUT
R1
)
â¢
VOUT
= (20V â 3.3V) ⢠3.3V = 10mW
5.5k
The respective values for Channel 2 are R1 = 3.66k,
R2 = 1.57k; and PLOSS R1 = 8mW.
Burst Mode operation is chosen for high light load efï¬ciency
(Figure 15) 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 (8mΩ). At heavier loads, DCR sensing provides
higher efï¬ciency.
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
=
3.3V
20V
(5)2
[1+
(0.005)(50°C
â
25°C)]
â¢
(0.023Ω)
+
(20V)2
â
ââ
5A
2
â
â â
(2Ω)(100pF)
â¢
â¡
â£â¢
5
1
â 2.3
+
1
2.3
â¤
â¦â¥
(500kHz)
=
186mW
100
10
90
EFFICIENCY
80
1
70
POWER LOSS
60
0.1
50
40
0.01
0.1
1
LOAD CURRENT (mA)
DCR
8mΩ
0.01
10
3865 F16
Figure 15. Design Example Efï¬ciency vs Load
3865fb
32
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