LTC3850-1_15 Datasheet, PDF (29/38 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down Switching Controller

LTC3850/LTC3850-1
APPLICATIONS INFORMATION
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
)
=
7mΩ
26.4mΩ
= 0.26;
and
7mΩ
39.6mΩ
≅
0.18
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
30mΩ • 0.1µF
=
1.1k
For channel 1, the DCR_SENSE filter/divider values are:
R1= ^R1||R2 = ^1.1k ≅ 6.19k;
R_D 0.18
R2
=
R1 • R_D
1− R_D
=
6.19k • 0.18
1− 0.18
≅
1.33k
The power loss in R1 at the maximum input voltage is:
P_LOSSR1= ^(VIN(MAX)
− V_OUT ) • V_OUT
R1
=
(20V
− 3.3V)
6.19k
•
3.3V
=
9mW
100
10
DCR
90
7mΩ
80
1
DCR
70
60
0.1
50
40
0.01
EFFICIENCY
POWER LOSS
0.01
0.1
1
10
LOAD CURRENT (mA)
38501 F15
Figure 15. Design Example Efficiency vs Load
The respective values for Channel 2 are R1 = 4.12k, R2 =
1.5k; 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 (7mΩ). 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
A short-circuit to ground will result in a folded back cur-
rent of:
I_SC
=
(1/ 3)50mV
0.007Ω
–
1
2


90ns(20V)
3.3µH 
=
2.1A
with a typical value of R_DS(ON) and d = (0.005/°C)(20)
= 0.1. The resulting power dissipated in the bottom
MOSFET is:
P_SYNC
=
20V – 3.3V
20V
(2.1A
)²
(1.125)(0.016Ω)
= 66mW
which is less than under full-load conditions.
C_IN is chosen for an RMS current rating of at least 2A at
temperature assuming only channel 1 or 2 is on. C_OUT 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:
V_ORIPPLE = R_ESR (∆I_L) = 0.02Ω(1.5A) = 30mV_P–P
38501fc
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