LX1673 查看數據表（PDF） - Microsemi Corporation
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LX1673 Datasheet PDF : 13 Pages
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LX1673
TM
®
High Frequency PWM Regulator
PRODUCTION DATA SHEET
APPLICATION NOTE (CONTINUED)
The SS pin voltage can be expressed as:
( ) V = V ref 1 − e − t/RSSCSS
SS
Where RSS and CSS are the soft-start resistor and capacitor.
The current required to charge the output capacitor during the soft
start interval is.
Iout = Cout dVss
dt
Taking the derivative with respect to time results in
Iout = VrefCout e− t/R SSCSS
RssCss
and at t=0
Im ax = VrefCout
RssCss
The required inductor current for the output capacitor to follow
the soft start voltage equals the required capacitor current plus the
load current. The soft-start capacitor should be selected to
provide the desired power on sequencing and insure that the
overall inductor current does not exceed its maximum allowable
rating.
Values of CSS equal to 0.1µf or greater are unlikely to result in
saturation of the output inductor unless very large output
capacitors are used.
OVER-CURRENT PROTECTION
Current limiting occurs at current level ICL when the voltage
detected by the current sense comparator is greater than the
current sense comparator threshold, VTRIP (300mV).
I ×R +I ×R = V
CL
DS(ON) SET
SET
TRIP
So,
V −I ×R
300 mV− I × R
R
= TRIP
CL
= DS(ON)
CL
DS(ON)
SET
I
50 µA
SET
Example:
For 10A current limit, using FDS6670A MOSFET ( 10mΩ
RDS(ON)):
R SET
=
0.3 −10 × 0.010
50 ×10−6
=
4K Ω
Note: Maximum RSET is 6KΩ . Any resistor 6KΩ or greater will
not allow startup since ICL will equal zero (50µA x 6KΩ =
300mV).
At higher PWM frequencies or low duty cycles where the upper
gate drive is less than 350nS wide the 350nS delay for current
limit enable may result in current pulses exceeding the desired
current limit set point. If the upper MOSFET on time is less than
350nS and a short circuit condition occurs the duty cycle will
increase, since VOUT will be forced low. The current limit circuit
will be enabled when the upper gate drive exceeds 350nS although
the actual peak current limit value will be higher than calculated
with the above equation.
Short circuit protection still exists due to the narrow pulse width
even though the magnitude of the current pulses will be higher than
the calculated value.
If OCP is not desired connect both VSX and VCX to VCC. Do not
leave them floating.
OUTPUT DISABLE
The LX1673 PWM MOSFET driver outputs are shut off by
pulling the disable (DIS) pin above 1.2V. There is a 80KΩ pull
down resistor on this input.
The LDO voltage regulator has its own Disable pin (LDDIS) for
control of this output voltage. Pulling this pin above 3 V disables
the LDO. There is a 100KΩ pull down resistor on this input.
PROGRAMMING THE OUTPUT VOLTAGE
The output Voltage is sensed by the feedback pin (FBX) which is
compared to a 0.8V reference. The output voltage can be set to any
voltage above 0.8V (and lower than the input voltage) by means of
a resistor divider (see Figure 1).
V = V (1 + R /R )
OUT
REF
12
Note: This equation is simplified and does not account for error
amplifier input current. Keep R1 and R2 close to 1KΩ (order of
magnitude).
DDR VTT TERMINATION VOLTAGE
Double Data Rate (DDR) SDRAM requires a termination
voltage (VTT) in addition to the line driver supply voltage (VDDQ)
and receiver supply voltage (VDD).
VTT for DDR memory can be generated with the LX1673 by
using the positive input of the phase 2 error amplifier RF2 as a
reference input from an external reference voltage VREF which is
defined as one half of VDDQ. Using VREF as the reference input
will insure that all voltages are correct and track each other as
specified in the JEDEC (EIA/JESD8-9A) specification. The phase 2
output will then be equal to VREF and track the VDDQ supply as
required.
When an external reference is used the connection between the
error amplifier positive input and the Soft Start pin is lost and Soft
Start will not function. It is recommended that the external
reference voltage have an R-C time constant that will be long
enough to allow the output capacitor to charge slowly.
See Microsemi Application Note 17 for more details
Copyright © 2004
Rev 1.0, 3/18/2005
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 10
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