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LT3514EFE#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26
LT3514
13
3514fa
For more information www.linear.com/LT3514
APPLICATIONS INFORMATION
peak inductor current. Provided that the inductor does not
saturate and that the switch current remains below 2A
(SW1, SW4) or below 4A (SW3), operation above V
IN(PS)
is safe and will not damage the part. For a more detailed
discussion on minimum on-time and pulse-skipping, re
-
fer to the Applications Information section of the LT3505
data sheet.
Avoid starting up the LT3514 at input voltages greater
than 36V, as the LT3514 must simultaneously conduct
maximum currents at high V
IN
. The maximum operating
junction temperature of 125°C may be exceeded due to
the high instantaneous power dissipation.
Frequency Selection
The maximum frequency that the LT3514 can be pro
-
grammed to is 2.5MHz. The minimum frequency is 250kHz.
The
switching
frequency can be programmed in two ways.
The first method is by tying a 1% resistor (R
T
) from the
RT/SYNC pin to ground. Table 1 can be used to select the
value of R
T
. The second method is to synchronize (sync)
the internal oscillator to an external clock. The external
clock must have a minimum amplitude from 0V to 1.5V
and a minimum pulse-width of 50ns.
Table 1. RT/SYNC Pin Resistance to Program Oscillator
Frequency
FREQUENCY (MHz) RT/SYNC PIN RESISTANCE (kΩ)
0.20 140
0.3 82.5
0.4 56.2
0.5 43.2
0.6 34.8
0.7 28.0
0.8 23.7
0.9 20.5
1.0 18.2
1.1 16.9
1.2 14.7
1.3 13.0
1.4 11.5
Figure 2. Driving the RT/SYNC Pin From a Port That
Is in a High Impedance State When it Is Inactive
Figure 3. Driving the RT/SYNC Pin from a Port That Is
in a Low Impedance State When it Is Inactive
FREQUENCY (MHz) RT/SYNC PIN RESISTANCE (kΩ)
1.5 10.7
1.6 9.76
1.7 8.66
1.8 8.06
1.9 7.32
2.0 6.81
2.1 6.34
2.2 6.04
2.3 5.62
2.4 5.36
2.5 4.99
In certain applications, the LT3514 may be required to be
alive and switching for a period of time before it begins
to receive a sync signal. If the sync signal is in a high
impedance state when it is inactive then the solution is to
simply tie an R
T
resistor from the RT/SYNC pin to ground
(Figure 2). The sync signal should be capable of driving the
R
T
resistor. If the sync signal is in a low impedance state
or an unknown state when it is inactive, then the solution
is to tie the R
T
resistor from the RT/SYNC pin to ground
and then to drive the RT/SYNC pin with the sync signal
through a 1nF capacitor as shown in Figure 3.
3514 F02
PORT
GND
LT3514
RT/SYNC
R
T
3514 F03
PORT
1nF
GND
LT3514
RT/SYNC
R
T
LT3514
14
3514fa
For more information www.linear.com/LT3514
APPLICATIONS INFORMATION
BOOST Regulator and SKY Pin Considerations
The on-chip boost regulator generates the SKY voltage
to be 4.85V above V
IN
. The SKY voltage is the source of
drive current for the buck regulators which is used to fully
saturate the power switch. The boost regulator requires
two external components: an inductor and a capacitor.
A good first choice for an inductor is given by:
L =
20.5µH
f
where f is in MHz.
Thus, for a 250kHz programmed switching frequency,
a good first choice for an inductor value is 82µH. For a
2.5MHz programmed switching frequency, a good first
choice for an inductor value is 8.2µH. These values will
ensure that each buck regulator will have sufficient drive
current to saturate the power switch in all applications
and under all operating conditions.
A user desiring a lower inductor current value can calculate
their optimum inductor size based on their output cur
-
rent requirements. Each buck regulator instantaneously
requires 20mA from the SKY pin per 1A of switch current.
The average current that each buck regulator draws from
the SKY pin is 20mA per 1A of switch current multiplied by
the duty cycle. So if all three buck regulators run at 100%
duty cycle
with channels 1 and 4 supplying 1A of output
current and channel 3 supplying 2A of output current, then
the SKY pin should be able to source 80mA. However if
each channel runs at 50% duty cycle then the SKY pin only
has to source 40mA. Alternatively if each channel runs
at 100% duty cycle but the output current requirements
are reduced by half, then again the SKY pin only has to
source 40mA. To summarize, the SKY pin output current
requirement is calculated from the following equation:
I
SKY
=
I
OUT1
• V
OUT1
+ I
OUT3
• V
OUT3
+ I
OUT4
• V
OUT4
( )
50 • V
IN
where I
OUTX
is the desired output current from Channel
X, V
OUTX
is the programmed output voltage of Channel X,
and V
IN
is input voltage.
Once the SKY pin output current requirement is deter-
mined, the inductor value can be calculated based on
the maximum tolerable inductor current ripple from the
following equation:
L =
V
IN
• DC5
2 • f
SW
• 0.3 • 1− 0.25 • DC5
( )
− I
SKY
where f
SW
is the programmed switching frequency and
DC5 is the boost regulator duty cycle, given by: DC5 =
5V/(V
IN
+ 5V).
For a 1MHz application, with V
IN
= 12V, V
OUT1
= 5V, V
OUT3
= 2.5V, V
OUT4
= 1.8V, I
OUT1,4
= 1A, I
OUT3
= 2A, and the
required SKY pin current is 20mA and the inductor value
is 6.8µH.
Soft-Start/Tracking
The RUN/SS pin can be used to soft-start the correspond
-
ing channel, reducing the maximum input current during
start-up. The RUN/SS pin is pulled up through a 1
µA current
source to about 2.1V. A capacitor can be tied to the pin to
create a voltage ramp at this pin. The buck regulator will
not switch while the RUN/SS pin voltage is less than 0.1V.
As the RUN/SS pin voltage increases above 0.1V, the chan
-
nel will begin switching and the FB pin voltage will track
the RUN/SS pin voltage (offset by 0.1V), until the RUN/SS
pin voltage is greater than 0.8V + 0.1V. At this point the
output voltage will be at 100% of it’
s programmed value
and the FB pin voltage will cease to track the RUN/SS
pin voltage and remain at 0.8V (the RUN/SS pin will
continue ramping up to about 2.1V with no effect on the
output voltage).
The ramp rate can be tailored so that the
peak start up current can be reduced to the current that
LT3514
15
3514fa
For more information www.linear.com/LT3514
Figure 5. Circuit to Prevent Switching When V
IN
< 10V, with 700mV of Hysteresis
APPLICATIONS INFORMATION
is required to regulate the output, with little overshoot.
Figure 4 shows the start-up waveforms with and without
a soft-start capacitor (C
SS
) on the RUN/SS pin.
Undervoltage Lockout
The LT3514 prevents switching when the input voltage
decreases below 3.2V. Alternatively, the EN/UVLO pin
can be used to program an undervoltage lockout at input
voltages exceeding 3.2V by tapping a resistor divider from
V
IN
to EN/UVLO as shown in Figure 5.
The rising threshold on the EN/UVLO pin is 1.44V. The
falling threshold on the EN/UVLO pin is 1.33V. When EN/
UVLO is rising and less than 1.44V then the EN/UVLO pin
sinks 1.3µA of current. This 1.3µA current can be used to
program additional hysteresis on the EN/UVLO pin. For the
circuit in Figure 5, R1 can be determined from:
R1=
V
IN,HYSTERESIS
−
0.11
1.33
V
IN,FALLING
( )
1.3µA
where V
IN,HYSTERESIS
is the desired amount of hysteresis
on the input voltage and V
IN,FALLING
is the desired input
voltage threshold at which the part will shut down. Notice
that for a given falling threshold (V
IN,FALLING
), the amount
of hysteresis (V
IN,HYSTERESIS
) must be at least:
V
IN,HYSTERESIS
>
0.11
1.33
• V
IN,FALLING
( )
R2
20.5k
R1
133k
GND
LT3514
EN/UVLO
V
IN
V
IN
3514 F05
NOT
SWITCHING
SWITCHING
V
IN, FALLING
= 10V
V
IN
(V)V
IN
(V)
9 10 11 12
V
IN, RISING
= 11V
Figure 4a. Inductor Current Waveform During
Start-Up without a Soft-Start Capacitor
Figure 4b. Inductor Current Waveform During
Start-Up with a 1nF Soft-Start Capacitor (C
SS
)
100µs/DIV
3514 F04a
I
L
0.5A/DIV
V
OUT
2V/DIV
100µs/DIV
3514 F04b
I
L
0.5A/DIV
V
OUT
2V/DIV
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LT3514EFE#TRPBF

Mfr. #:
Buy LT3514EFE#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Triple Step-Down Switching Regulator with 100% Duty Cycle Operation
Lifecycle:
New from this manufacturer.
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