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LTM4639 Arkusz danych(PDF) 13 Page - Linear Technology

Numer części LTM4639
Szczegółowy opis  Ultrathin, Triple Output, Step-Down 關Module Regulator for DDR-QDR4 Memory
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Producent  LINER [Linear Technology]
Strona internetowa  http://www.linear.com
Logo LINER - Linear Technology

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LTM4632
13
4632fc
For more information www.linear.com/LTM4632
APPLICATIONS INFORMATION
Application Note 77 provides a detailed explanation of
multiphase operation. The input RMS ripple current can-
cellation mathematical derivations are presented, and
a graph is displayed representing the RMS ripple cur-
rent reduction as a function of the number of interleaved
phases. Figure 4 shows this graph.
Channel 1 Output Voltage Tracking and Soft-Start
The TRACK/SS pin provides a means to either soft-start
the Channel 1 regulator or track it to a different power
supply. A capacitor on the TRACK/SS pin will program
the ramp rate of the channel 1 output voltage. An internal
1.2µA current source will charge up the external soft-start
capacitor towards INTVCC voltage. When the TRACK/SS
voltage is below 0.6V, it will take over the internal 0.6V
reference voltage to control the output voltage. The total
soft-start time can be calculated as:
tSS = 0.6 •
CSS
1.2µA
where CSS is the capacitance on the TRACK/SS pin.
Forced continuous mode are disabled during the soft-
start process.
Channel 1 output voltage tracking can also be programmed
externally using the TRACK/SS pin. The output can be
tracked up and down with another regulator. Figure 5 and
Figure 6 show an example waveform and schematic of a
Ratiometric tracking where the slave regulator’s output
slew rate is proportional to the master’s.
Since the slave regulator’s TRACK/SS is connected to
the master’s output through a RTR(TOP)/RTR(BOT) resistor
divider and its voltage used to regulate the slave output
voltage when TRACK/SS voltage is below 0.6V, the slave
output voltage and the master output voltage should sat-
isfy the following equation during the start-up.
VOUT(SL)
RFB(SL)
RFB(SL) +60.4k
=
VOUT(MA)
RTR(BOT)
RTR(TOP) +RTR(BOT)
The RFB(SL) is the feedback resistor and the RTR(TOP)/
RTR(BOT) is the resistor divider on the TRACK/SS pin of
the slave regulator, as shown in Figure 6.
Following the upper equation, the master’s output slew
rate (MR) and the slave’s output slew rate (SR) in Volts/
Time is determined by:
MR
SR
=
RFB(SL)
RFB(SL) +60.4k
RTR(BOT)
RTR(TOP) +RTR(BOT)
For example, VOUT(MA) = 1.5V, MR = 1.5V/1ms and
VOUT(SL) = 1.2V, SR = 1.2V/1ms. From the equation, we
could solve out that RTR(TOP) = 60.4k and RTR(BOT) = 40.2k
is a good combination for the Ratiometric tracking.
TIME
SLAVE OUTPUT
MASTER OUTPUT
4632 F05
Figure 5. Output Ratiometric Tracking Waveform
The TRACK pins will have the 1.2µA current source on
when a resistive divider is used to implement tracking on
that specific channel. This will impose an offset on the
TRACK pin input. Smaller values resistors with the same
ratios as the resistor values calculated from the above
equation can be used. For example, where the 60.4k is
used then a 6.04k can be used to reduce the TRACK pin
offset to a negligible value.
The Coincident output tracking can be recognized as a
special Ratiometric output tracking which the master’s
output slew rate (MR) is the same as the slave’s output
slew rate (SR), as waveform shown in Figure 7.


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