LTC3811_15 Datasheet, PDF (37/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
APPLICATIONS INFORMATION
L1 L2
R_S1 R_S2
SENSE1⁺
SENSE2⁺
SENSE1^–
SENSE2^–
+
–^gm
CSAMP
CSOUT
R_AVP
C_OUT
DIFFIN⁺ _80k
DIFFIN^{– 80k}
DIFFOUT
SGND
80k
+
–
DIFFAMP
80k
COMP
C2
R2 R3
R1
SGND
C1
FB ^0.6V
+ ^EA
–
3811 F26
Figure 26. Simplified LTC3811 Voltage Positionig Block Diagram
Figure 27 illustrates the load step response for the circuit
in Figure 30, with voltage positioning.
A Design Example
As a design example consider one channel of a 2-phase
single output supply. Assume V_IN = 4.5V to 14V, V_OUT =
1.5V, I_MAX = 30A(15A per phase) and f = 500kHz.
In order to achieve the best output accuracy, 1% resistors
(or better) should be used in the divider that programs the
output voltage. Choosing 600μA for the divider current,
R2 = 1.5k and R1 = 1k. The nominal output voltage will
therefore be 1.50V.
For the input and output conditions given above, the steady
state minimum on-time for this application at V_IN = 14V
will be approximately:
t_ON(MIN)
=
V_OUT
V_IN(MAX)
•
f
=
1.5V
14V • 500kHz
=
214ns
V_OUT
50mV/DIV
AC COUPLED
I_L1
5A/DIV
I_L2
5A/DIV
V_IN = 12V
50μs/DIV
V_OUT = 1.5V (2-PHASE)
I_LOAD = 0A TO 15A
3811 G03
Figure 27. Load Step Response for the Circuit in Figure 30
To program 500kHz operation, float the PLL/LPF pin.
The inductor value in this design is chosen assuming
50% ripple current. The highest ripple current occurs at
maximum input voltage.
L=
V_OUT
f • ΔI_L
⎡
• ⎢1–
⎣⎢
V_OUT
V_{IN(MAX )}
⎤
⎥
⎦⎥
=
1.5V
500kHz • 0.5
• 15A
•
⎡⎣⎢_1–
1.5V ⎤
14V _⎦⎥
=
0.36μH
A 0.4μH inductor will result in 6.7A of ripple current, or
45%. Assuming a value for the current limit value 30%
greater than the maximum load current, then I_MAX = 1.3 •
15A = 19.5A. The peak inductor current will be the maxi-
mum DC value plus one half the ripple current, or:
I_L(PK)
=
19.5A
+
1
2
•
6.7A
=
22.9A
This represents the minimum saturation current rating for
the inductor. For this application, a Vitec 59P9875 inductor
was chosen. This inductor has a room temp saturation
current rating of 23A and a DCR of 0.32mΩ.
To maintain good cycle-by-cycle control of the inductor
current and still have good efficiency, a 1.5mΩ, 1W sense
resistor from Panasonic (ERJ-M1WTJ1M5U) connected
in series with the inductor is used for current sensing.
With a maximum peak inductor current of 22.9A, the peak
sense voltage will be:
V_SENSE(PK) = 22.9A • 0.0015Ω = 34.4mV
3811f
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