LTC3738CUHF Linear Technology, LTC3738CUHF Datasheet - Page 28

• Manufacturer Part Number: LTC3738CUHF
• Description: IC CTRLR SW REG 3PH STPDWN 38QFN
• Manufacturer: Linear Technology

Specifications of LTC3738CUHF

• Applications: Controller, Intel VRM9, VRM10
• Voltage - Input: 3.8 ~ 36 V
• Number Of Outputs: 1
• Voltage - Output: 0.84 ~ 1.6 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 38-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
LTC3738
and the worst-case input RMS ripple current for a three
stage design results in peaks at 1/6, 1/2, and 5/6 of the
input voltage. The peaks, however, are at ever decreasing
levels with the addition of more phases. A higher effective
duty factor results because the duty factors “add” as long
as the currents in each stage are balanced. Refer to AN19
for a detailed description of how to calculate RMS current
for the single stage switching regulator.
Figure 5 illustrates the RMS input current drawn from the
input capacitance versus the duty cycle as determined by
the ration of input and output voltage. The peak input RMS
current level of the single phase system is reduced by 2/3
in a 3-phase solution due to the current splitting between
the three stages.
The output ripple current is reduced significantly when
compared to the single phase solution using the same
inductance value because the V
term from the stages that has their bottom MOSFETs on
28
Figure 11. Single and Polyphase Current Waveforms
SW1 V
SW2 V
SW3 V
I
I
SW V
COUT
COUT
I
I
CIN
CIN
I
I
I
L1
L2
L3
U
SINGLE PHASE
TRIPLE PHASE
U
OUT
W
/L discharge currents
3738 F11
U
subtract current from the (V
resulting from the stage which has its top MOSFET on. The
output ripple current for a 3-phase design is:
The ripple frequency is also increased by three, further
reducing the required output capacitance when V
as illustrated in Figure 3.
Efficiency Calculation
To estimate efficiency, the DC loss terms include the input
and output capacitor ESR, each MOSFET R
tor resistance R
forward drop of the Schottky rectifier at the operating
output current and temperature. Typical values for the
design example given previously in this data sheet are:
The main MOSFET is on for the duty factor V
the synchronous MOSFET is on for the rest of the period
or simply (1 – V
small, the AC loss in the inductor can be made small if a
good quality inductor is chosen. The average current,
I
below is not exact but should provide a good technique
for the comparison of selected components and give a
OUT
Main MOSFET R
Sync MOSFET R
C
C
R
R
V
V
V
I
δ = 0.5%°C
N = 3
f = 400kHz
I
P-P
MAX
INESR
OUTESR
SCHOTTKY
OUT
IN
L
SENSE
is used to simplify the calaculations. The equation
= 2mΩ
= 12V
=
= 45A
= 1.3V
( )( )
V
= 20mΩ
= 3mΩ
f L
OUT
= 3mΩ
= 0.8V at 15A (0.7V at 90°C)
(
OUT
1 3
L
, the sense resistance R
–
DS(ON)
DS(ON)
/V
DC
IN
). Assuming the ripple current is
)
= 13.5mΩ (18mΩ at 90°C)
= 4mΩ (5.3mΩ at 90°C)
CC
V
IN
– V
>
3
OUT
V
OUT
)/L charging current
SENSE
DS(ON)
OUT
CC
/V
< 3V
and the
, induc-
IN
and
OUT
3738f