LTC3883 Datasheet, PDF (58/112 Pages) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

LTC3883/LTC3883-1
Applications Information
The maximum power loss in R0 is related to the duty
cycle, and will occur in continuous mode at the maximum
input voltage:
( ) _PLOSSR1=
V_IN(MAX) – V_OUT
R1
• V_OUT
=
(20
– 1.8) •1.8
1.37k
=
23.91mW
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.
The power dissipation on the topside MOSFET can be eas-
ily 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:
P_MAIN
=
1.8V
20V
•
(17.25)²
•
1+
(0.005)(50°C
–
25°C)
•
0.01Ω
+
(20V)²
(8.695A
)
•


5
1
– 2.3
+
1
2.3


(75pF)(500kHz) = 0.406W
The loss in the bottom side MOSFET is:
P_SYNC
=
(20V – 1.8V
20V
)
•
(17.25A)²
•
1+(0.005)(50°C – 25°C) • 0.003Ω
= 0.913W
Both MOSFETS have I²R losses while the P_MAIN equation
includes an additional term for transition losses, which
are highest at high input voltages.
C_IN is chosen for an RMS current rating of:
C_IN
Re quired
I_RMS
=
17.25
20
(1.8) • (20 – 1.8)^1/2
= 4.9A
at temperature. C_OUT is chosen with an ESR of 0.006Ω 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
V_ORIPPLE = R(∆I_L) = 0.006Ω • 5.5A = 33mV.
Connecting the USB to I²C/SMBus/PMBus
Controller to the LTC3883 In System
The LTC USB to I²C/SMBus/PMBus controller can be
interfaced to the LTC3883 on the user’s board for pro-
gramming, telemetry and system debug. The controller,
when used in conjunction with LTpowerPlay, provides a
powerful way to debug an entire power system. Faults are
quickly diagnosed using telemetry, fault status commands
and the fault log. The final configuration can be quickly
developed and stored to the LTC3883 EEPROM.
Figure 29 illustrates the application schematic for
powering, programming and communication with one or
more LTC3883s via the LTC I²C/SMBus/PMBus controller
regardless of whether or not system power is present. If
system power is not present the dongle will power the
LTC3883 through the V_DD33 supply pin. To initialize the
part when V_IN is not applied and the V_DD33 pin is powered
use global address 0x5B command 0xBD data 0x2B
followed by address 0x5B command 0xBD data 0xC4.
The part can now be communicated with, and the project
file updated. To write the updated project file to the NVM
issue a STORE_USER_ALL command. When VIN is applied,
a MFR_RESET must be issued to allow the PWM to be
enabled and valid ADCs to be read.
Because of the controllers limited current sourcing capabil-
ity, only the LTC3883s, their associated pull-up resistors
and the I²C pull-up resistors should be powered from the
ORed 3.3V supply. In addition any device sharing the I²C
bus connections with the LTC3883 should not have body
diodes between the SDA/SCL pins and their respective
3883f
58