LT3505 LINER [Linear Technology], LT3505 Datasheet - Page 12

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

LT3501
applicaTions inForMaTion
Choosing the Output Voltage
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
R2 should be 10k or less to avoid bias current errors. Refer-
ence designators refer to the Block Diagram in Figure 1.
Choosing the Switching Frequency
The LT3501 switching frequency is set by resistor R3 in
Figure 1. The R
Setting resistor R3 sets the current in the R
which determines the oscillator frequency as illustrated
in Figure 2.
The switching frequency is typically set as high as pos-
sible to reduce overall solution size. The LT3501 employs
techniques to enhance dropout at high frequencies but
efficiency and maximum input voltage decrease due to
switching losses and minimum switch on-times. The
maximum recommended frequency can be approximated
by the equation:
where V
Figure 2), V
and t
maximum load current.
12
R1= R2 •
Frequency (Hz) =
ON(MIN)
D
is the forward-voltage drop of the catch diode (D1
SW

 
in the minimum on-time of the switch, all at
0.8V
V
T
is the voltage drop of the internal switch,
/SYNC pin is internally regulated at 0.975V.
OUT
– 1
V
Table 1. Efficiency and Size Comparisons for Different R
†
*Inductor and capacitor values chosen for stability and constant ripple current.
V

 
IN
FREQUENCY
IN(MAX)
V
OUT
1.2MHz
1.0MHz
750kHz
500kHz
250kHz
– V
SW
is defined as the highest input voltage that maintains constant output voltage ripple.
+ V
+ V
D
D
R
T
20.5k
26.7k
38.3k
61.9k
133k
•
/SYNC
t
ON(MIN)
1
T
V
/SYNC pin
EFFICIENCY
VIN1/2
79.0%
80.9%
81.2%
82.0%
83.9%
= 12V
V
IN(MAX)
The following example along with the data in Table 1
illustrates the trade-offs of switch frequency selection.
Example:
Input Voltage Range
Once the switching frequency has been determined, the
input voltage range of the regulator can be determined. The
minimum input voltage is determined by either the LT3501’s
minimum operating voltage of ~2.8V, or by its maximum
16
18
22
24
24
V
Temperature = 0°C to 85°C
t
graph), V
Max Frequency =
R
ON(MIN)
IN
Figure 2. Frequency and Phase vs R
T
†
/SYNc ~ 42k (Figure 2)
= 25V, V
1600
1400
1200
1000
800
600
400
200
100
RT/SYNC
1.5µH
2.2µH
3.3µH
4.7µH
10µH
= 200ns (85°C from Typical Characteristics
L*
D
0
= 0.6V, V
OUT
20
Values. 3.3V Output
FREQUENCY
100µF
= 3.3V, I
22µH
47µH
47µF
47µF
C*
40
RESISTANCE (k )
25 – 0.4 + 0.6
SW
60
3.3 + 0.6
= 0.4V (85°C)
172mm
OUT
63mm
66mm
66mm
66mm
80
C + L
= 2.5A,
100 120 140
2
2
2
2
2
PHASE
T
/SYNC Resistance
•
3501 F02
200e-9
1
190
185
180
175
170
165
160
155
150
~ 750 kHz
3501fd