LTC3876_15 Datasheet, PDF (37/48 Pages) Linear Technology – Dual DC/DC Controller for DDR Power with Differential VDDQ Sensing and 50mA VTT Reference

LTC3876
APPLICATIONS INFORMATION
The minimum on-time for channel 1 VDDQ is:
t_ON(MIN)
=
V_OUT
V_{IN(MAX )}
•
f
=
1.5V
14V • 400kHz
=
268ns
The minimum on-time for channel 2 VTT is:
t_ON(MIN)
=
V_OUT
V_{IN(MAX )}
•
f
=
0.75V
14V • 400kHz
=
134ns
Set the channel 1 VDDQ inductor value L1 to give 35%
ripple current at the maximum load to 20A for the maxi-
mum V_IN of 14V using the adjusted operating frequency.
L1=
V_OUT
( f)(I_RIPPLE)
⎛
⎜1−
⎝
V_OUT
⎞
⎟
V_{IN(M AX )}⎠
=
(
1.5V
400kHz ) ( 35%
t
20A
)
⎛
⎜1−
⎝
1.5
14
⎞
⎟
⎠
=
0.47μH
Set the channel 2 VTT inductor value L2 to give 35% ripple
current at the maximum load to 10A for the maximum V_IN
of 14V using the adjusted operating frequency.
L1=
V_OUT
( f)(I_RIPPLE)
⎛
⎜^1−
⎝
V_OUT
⎞
⎟
V_{IN(M AX )}⎠
=
0.75V
( 400kHz ) ( 37.5%
t
10A
)
⎛
⎜^1−
⎝
0.75^⎞⎟
14 ⎠
=
0.47μH
Choose the nearest standard value of 0.47μH for L2, which
will result in 37.5% ripple current.
The resulting channel 1 VDDQ maximum ripple current is:
ΔI_L1 =
1.5V
( 400kHz )(0.47μH )
⎛
⎜1−
⎝
1.5V^⎞⎟
14V ⎠
=
7.12A
The resulting channel 2 VTT maximum ripple current is:
ΔI_L2
=
0.75V
(400kHz)(0.47μH
)
⎛
⎜1−
⎝
0.75V^⎞⎟
14V ⎠
=
3.78A
For Figure 11, standard demo board the current limit is set
using sense resistors. The V_RNG is grounded which results
in a maximum V_SENSE voltage across the R_SENSE resistor
of 30mV. If we assume 50% over the nominal output of
20A this gives a starting point of 1mΩ for channel 1 VDDQ
and 2mΩ for channel 2 VTT.
Channel 1 VDDQ current limit for 1mΩ R_SENSE.
ILIMIT_VDDQ
=
V_SENSE
R_SENSE
+
ΔI_L
2
=
30mV
1mΩ
+
7.12A
2
=
33.5A
Channel 2 VTT current limit for 2mΩ R_SENSE.
ILIMIT_VTT
=
V_SENSE
R_SENSE
+
ΔI_L
2
=
30mV
2mΩ
+
3.78A
2
=
16.9A
In high power applications, DCR current sensing is often
preferred to R_SENSE in order to maximize efficiency. The
inductor model is selected based on its inductor and DCR
value. The Würth WE7443330047 with a rated current of
20A, a saturation current of 47A and DCR of 0.8mΩ is
chosen for channel 1 VDDQ. The Würth WE7443340047
with a rated current of 19A, a saturation current of 32A
and DCR of 1.72mΩ is chosen for channel 2 VTT. The DCR
demo board design is Figure 13.
In this design example V_RNG was grounded to produce an
internal default value of 30mV on V_SENSE.
Channel 1 VDDQ DCR Current limit:
IL
IMIT_VDDQ
=
V_SENSE
R _SENSE
+
Δ I_L
2
=
30mV
+ ^7.12A
0.8mΩ • (1+(100°C – 25°C) • 0.4% / ^°C) 2
= 32.4A
Channel 2 VTT DCR Current Limit:
ILIM IT_{V T T}=
V_SENSE
R _SENSE
+
Δ IL
2
=
30mV
+ ^3.78A
1.72mΩ • (1+(100°C – 25°C) • 0.4% / °C) 2
= 15.3A
3876f
37