Datasheet

LTC3880/LTC3880-1
58
3880fc
For more information www.linear.com/LTC3880
APPLICATIONS INFORMATION
the resistor divider between VDD25 FREQ_CFG and SGND
with R
TOP
= 20k and R
BOTTOM
= 12.7k. The address is set
to XF where X is the MSB stored in NVM.
The following parameters are set as a percentage of the
output voltage if the resistor configuration pins are used
to determined output voltage:
n
VOUT_OV_FAULT_LIMIT .................................... +10%
n
VOUT_OV_WARN .............................................. +7.5%
n
VOUT_MAX....................................................... +7.5%
n
VOUT_MARGIN_HI ..............................................+5%
n
POWER_GOOD_ON .............................................–7%
n
POWER_GOOD_OFF ............................................–8%
n
VOUT_MARGIN_LO .............................................–5%
n
VOUT_UV_WARN ..............................................–6.5%
n
VOUT_UV_FAULT_LIMIT ......................................–7%
All other user defined parameters must be programmed
into the NVM. The GUI can be utilized to quickly set up
the part with the desired operating parameters.
The inductance values are based on a 35% maximum
ripple current assumption (5.25A for each channel). The
highest value of ripple current occurs at the maximum
input voltage:
L =
V
OUT
f • ∆I
L(MAX)
1–
V
OUT
V
IN(MAX)








Channel 0 will require 1.05µH and channel 1 will require
0.624µH. The nearest standard values are 1µH and 0.68µH
respectively. At the nominal input the ripple will be:
∆I
L(NOM)
=
V
OUT
f •L
1–
V
OUT
V
IN(NOM)








Channel 0 will have 4.79A (32%) ripple, and channel 1 will
have 5.5A (30%) ripple. The peak inductor current will be
the maximum DC value plus one-half the ripple current
or 17.39A for channel 0 and 17.75A for channel 1. The
minimum on time occurs on channel 1 at the maximum
V
IN
, and should not be less than 90ns:
t
ON(MIN)
=
V
OUT
V
IN(MAX)
• f
=
1.8V
20V 500kHz
( )
=180ns
The Vishay IHLP4040DZ-11 1µH (2.3mΩ DCR
TYP
at 25°C)
channel 0 and the VishayIHLP4040DZ-11 0.56µH (1.61mΩ
DCR
TYP
at 25°C) channel 1 are chosen.
Assuming the temperature measurement of the inductor
temperature is accurate and C1 is set to 0.2µF, R
D
is infinite
and removed from the equations.
R0 =
L
DCR at 25°C
( )
•C1
=
1µH
2.3mΩ • 0.22µF
= 2k
The maximum power loss in R0 is related to the duty
cycle, and will occur in continuous mode at the maximum
input voltage:
P
LOSS
R0 =
V
IN(MAX)
– V
OUT
( )
• V
OUT
R1
=
20 – 3.3
( )
•3.3
2k
= 27.55mW
The respective values for channel 1 are R1 = 2k, R2 is
open and P
LOSS
R1 = 20.73mW.
The current limit will be set 20% higher than the peak
value to assure variation in components and noise in the
system do not limit the average current.
V
ILIMIT
= I
PEAK
• R
DCR(MAX)
= 17.39A • 2.5mΩ = 43mV
The closest V
ILIMIT
setting is 42.9mV or 46.4mV. The values
are entered with the IOUT_OC_FAULT_LIMIT command.
Based on expected variation and measurement in the lab
across the sense capacitor the user can determine the
optimal setting. For channel 1 the V
ILIMT
value is 28.6mV.
The closest value Iis 28.6mV.
The power dissipation on the top side MOSFET can be
easily estimated. Choose a RENESAS RJK0305DPB topside
MOSFET. R
DS(ON)
= 10mΩ, C
MILLER
= 75pF. At maximum
input voltage with T estimated = 50°C and a bottom side
MOSFET a RENESAS RJK0330DPB, R
DS(ON)
= 3mΩ: