LT3505 LINER [Linear Technology], LT3505 Datasheet - Page 13

• Manufacturer Part Number: LT3505
• Description: Monolithic Dual Tracking 3A Step-Down Switching
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
duty cycle. The duty cycle is the fraction of time that the
internal switch is on during a clock cycle. Unlike
fixed frequency regulators, the LT3501 will not switch off
at the end of each clock cycle if there is sufficient voltage
across the boost capacitor (C3 in Figure 1) to fully satu-
rate the output switch. Forced switch-off for a minimum
time will only occur at the end of a clock cycle when the
boost capacitor needs to be recharged. This operation
has the same effect as lowering the clock frequency for a
fixed off-time, resulting in a higher duty cycle and lower
minimum input voltage. The resultant duty cycle depends
on the charging times of the boost capacitor and can be
approximated by the following equation:
where B is 3A divided by the typical boost current from
the Electrical Characteristics table.
This leads to a minimum input voltage of:
where V
Figure 3 shows a typical graph of minimum input voltage
vs load current for the 3.3V and 1.8V application on the
first page of this data sheet. The maximum input voltage
is determined by the absolute maximum ratings of the V
and BST pins and by the frequency and minimum duty
cycle. The minimum duty cycle is defined as :
Maximum input voltage as:
Note that the LT3501 will regulate if the input voltage is
taken above the calculated maximum voltage as long as
maximum ratings of the V
However operation in this region of input voltage will exhibit
pulse skipping behavior.
DC
Dc
V
V
IN(MIN)
IN(MAX)
MIN
MAX
SW
= t
=
is the voltage drop of the internal switch.
=
ON(MIN)
=
1+
V
V
1
OUT
Dc
OUT
Dc
B
1
MAX
MIN
+ V
• Frequency
+ V
D
D
– V
IN
– V
and BST pins are not violated.
D
D
+ V
+ V
SW
SW
most
IN
Example:
V
= 25°C
V
specification), V
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
where f is frequency in MHz and L is in µH.
With this value the maximum load current will be ~3A,
independent of input voltage. The inductor’s RMS current
rating must be greater than your maximum load current
OUT
SW
Dc
Dc
L =
V
V
IN(MIN)
IN(MAX)
= 0.1V, B = 50 (from from boost characteristics
= 3.3V, I
MAX
MIN
Figure 3. Minimum Input Voltage vs Load Current
(
V
IN
6.0
5.5
5.0
4.5
4.0
3.5
3.0
= t
=
=
– V
=
1
1+
MIN(ON)
V
OUT
3.3 + 0.4
V
OUT
3.3 + 0.4
OUT
IN
1
0.200
D
0.98
50
= 3.3V
1
= 1A, Frequency = 1MHz, Temperature
• f
= 0.4V, t
)
10
= 98%
• V
• f = 0.200
CURRENT (mA)
OUT
– 0.4 + 0.1= 3.48V
– 0.4 + 0.1= 18.2V
RUNNING
START-UP
ON(MIN)
100
1000
= 200ns
3501 F03
10000
LT3501
13
3501fd