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LT3504IUFD#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT3504
10
3504fa
For more information www.linear.com/LT3504
FB Resistor Network
The output voltage is programmed with a resistor divider
connected from the output and the FB pin. Choose the 1%
resistor according to:
R1= R2 •
V
OUT
0.8V
− 1
⎛
⎝
⎜
⎞
⎠
⎟
A good value for R2 is 10kΩ, R2 should not exceed 20kΩ
to avoid bias current error.
Input Voltage Range
The input voltage range for LT3504 applications depends
on the output voltage and on the absolute maximum rat
-
ing of the V
IN
pin.
The minimum input voltage to regulate the output gener-
ally has to be at least 400mV greater than the greatest
programmed output voltage. The only exception is when
the largest programmed output voltage is less than 2.8V.
In this case the minimum input voltage is 3.2V
.
The absolute maximum input voltage of the LT3504 is
40V and the part will regulate output voltages as long
as the input voltage remains less than or equal to 40V.
However for constant-frequency operation (no pulse-
skipping) the maximum input voltage is determined by
the minimum on-time of the LT3504 and the programmed
switching frequency. The minimum on-time is the shortest
period of time that it takes the switch to turn on and off.
Therefore the maximum input voltage to operate without
pulse-skipping is:
V
IN(PS)
= [ (V
OUT
+ V
D
)/(f
SW
• t
ON(MIN)
) ] + V
SW
– V
D
where:
• V
IN(PS)
is the maximum input voltage to operate in
constant frequency operation without skipping pulses.
• V
OUT
is the programmed output voltage
• V
SW
is the switch voltage drop, at I
OUT
= 1A, V
SW
= 0.4V
• V
D
is the catch diode forward voltage drop, for an ap-
propriately sized diode, V
D
= 0.4V
• f
SW
is the programmed switching frequency
• t
ON(MIN)
is the minimum on-time, worst-case over
temperature = 110ns (at T = 125°C)
applicaTions inForMaTion
At input voltages that exceed V
IN(PS)
the part will continue
to regulate the output voltage up to 40V. However the
part will skip pulses (see Figure 1) resulting in unwanted
harmonics, increased output voltage ripple, and increased
peak inductor current. Provided that the inductor does not
saturate and that the switch current remains below 2A,
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, refer to the Applications Information
section of the LT3505 data sheet.
Avoid starting up the LT3504 at input voltages greater
than 36V, as the LT3504 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.
Figure 1a: The LT3504 Operating in Constant-Frequency
Operation (Below V
IN(PS)
), V
IN
= 26.5V, V
OUT
= 3.3V,
f
SW
= 2MHz, t
ON(MIN)
= 74ns at T = 25°C
Figure 1b.The LT3504 Operating in Pulse-Skipping Mode
(Above V
IN(PS)
), V
IN
= 27V, V
OUT
= 3.3V, f
SW
= 2MHz,
t
ON(MIN)
= 74ns at T = 25°C
2µs/DIV
3504 F01a
I
L
0.5A/DIV
V
SW
10V/DIV
2µs/DIV
3504 F01b
I
L
0.5A/DIV
V
SW
10V/DIV
LT3504
11
3504fa
For more information www.linear.com/LT3504
applicaTions inForMaTion
Frequency Selection
The maximum frequency that the LT3504 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 1.6V
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
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 LT3504 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
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
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.
3504 F02
PORT
GND
LT3504
RT/SYNC
R
T
3504 F03
PORT
1nF
GND
LT3504
RT/SYNC
R
T
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
LT3504
12
3504fa
For more information www.linear.com/LT3504
applicaTions inForMaTion
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 multiplied by the duty cycle. So if
all four buck regulators run at 100% duty cycle with each
channel supplying 1A of output current, then the SKY pin
should be able to source 80mA. However if each chan
-
nel 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 requirement is 0.5A per
channel instead of 1A, 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
OUT2
• V
OUT2
+
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
OUT2
= 3.3V, V
OUT3
= 2.5V, V
OUT4
= 1.8V, and all channels
supplying 1A of output current, the required SKY pin cur-
rent is 21mA and the inductor value is 6µH.
Soft-Start/T
racking
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 1µA 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
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.
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
3504 F04a
I
L
0.5A/DIV
V
OUT
2V/DIV
100µs/DIV
3504 F04b
I
L
0.5A/DIV
V
OUT
2V/DIV
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LT3504IUFD#PBF

Mfr. #:
Buy LT3504IUFD#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Quad 40V/1A Step-Down Switching Regulator with 100% Duty Cycle Operation
Lifecycle:
New from this manufacturer.
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