LTC3865 Datasheet, PDF (31/36 Pages) Linear Technology – Dual, 2-Phase Synchronous DC/DC Controller with Pin Selectable Outputs

LTC3865/LTC3865-1
Applications Information
With I_LIM floating, the equivalent R_SENSE resistor value
can be calculated by using the minimum value for the
maximum current sense threshold (44mV).
R_SENSE(EQUIV)
=
V_SENSE(MIN)
I_LOAD(MAX)
+
∆I_L(NOM)
2
=
44mV
5A
+
1.5A
2
@ 7.7mΩ
The equivalent R_SENSE is the same for Channel 2.
The Coiltronics (Cooper) HCP0703-2R2 (20mΩ DCR_MAX
at 20°C) and HCP0703-3R3 (30mΩ DCR_MAX at 20°C) are
chosen. At 100°C, the estimated maximum DCR values are
26.4mΩ and 39.6mΩ. The divider ratios are:
R_D
=
R_SENSE(EQUIV)
DCR_MAX at T_L(MAX)
=
7.7mΩ
26.4mΩ
= 0.3;
and
7.7mΩ
39.6mΩ
@
0.2
For each channel, 0.1µF is selected for C1.
R1||R2 =
(DCR_MAX
L
at
20°C) • C1 ⁼
2.2µH
20mΩ • 0.1µF
= 1.1k ; and
3.3µH _{= 1.1k}
30mΩ • 0.1µF
For channel 1, the DCR_SENSE filter/divider values are:
R1=
R1||R2
R_D
=
1.1k
0.2
@
5.5 k ;
R2 = ^{R1• RD} = ^{5.5k • 0.2} @ 1.37k
1−R_D 1− 0.2
The power loss in R1 at the maximum input voltage is:
P_LOSSR1=
(V_IN(MAX)
− V_OUT
R1
)
•
V_OUT
=
(20V
−
3.3V)
5.5k
•
3.3V
=
10mW
The respective values for Channel 2 are R1 = 3.66k,
R2 = 1.57k; and P_LOSS R1 = 8mW.
Burst Mode operation is chosen for high light load efficiency
(Figure 15) by floating 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 efficiency.
The power dissipation on the topside MOSFET can be easily
estimated. Choosing a Siliconix Si4816BDY dual MOSFET
results in: R_DS(ON) = 0.023Ω/0.016Ω, C_MILLER @ 100pF.
At maximum input voltage with T(estimated) = 50°C:
P_MAIN
=
3.3V
20V
(5)²
[¹⁺
(0.005)(50°C
–
25°C)_]
•
(0.023Ω)
+
(20V)²


5A
2


(2Ω)(100pF)
•


5
1
– 2.3
+
1
2.3


(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 Efficiency vs Load
3865f
31