Datasheet

LT3501
12
3501fd
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:
R1= R2 •
V
OUT
0.8V
– 1






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
T
/SYNC pin is internally regulated at 0.975V.
Setting resistor R3 sets the current in the R
T
/SYNC pin
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:
Frequency (Hz) =
V
OUT
+ V
D
V
IN
– V
SW
+ V
D
•
1
t
ON(MIN)
where V
D
is the forward-voltage drop of the catch diode (D1
Figure 2), V
SW
is the voltage drop of the internal switch,
and t
ON(MIN)
in the minimum on-time of the switch, all at
maximum load current.
The following example along with the data in Table 1
illustrates the trade-offs of switch frequency selection.
Example:
V
IN
= 25V, V
OUT
= 3.3V, I
OUT
= 2.5A,
Temperature = 0°C to 85°C
t
ON(MIN)
= 200ns (85°C from Typical Characteristics
graph), V
D
= 0.6V, V
SW
= 0.4V (85°C)
Max Frequency =
3.3 + 0.6
25 – 0.4 + 0.6
•
1
200e-9
~ 750 kHz
R
T
/SYNC ~ 42k (Figure 2)
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
Figure 2. Frequency and Phase vs R
T
/SYNC Resistance
Table 1. Efficiency and Size Comparisons for Different R
RT/SYNC
Values. 3.3V Output
FREQUENCY R
T
/SYNC
EFFICIENCY
V
VIN1/2
= 12V V
IN(MAX)
†
L* C* C + L
1.2MHz 20.5k 79.0% 16 1.5µH 22µH 63mm
2
1.0MHz 26.7k 80.9% 18 2.2µH 47µH 66mm
2
750kHz 38.3k 81.2% 22 3.3µH 47µF 66mm
2
500kHz 61.9k 82.0% 24 4.7µH 47µF 66mm
2
250kHz 133k 83.9% 24 10µH 100µF 172mm
2
†
V
IN(MAX)
is defined as the highest input voltage that maintains constant output voltage ripple.
*Inductor and capacitor values chosen for stability and constant ripple current.
RESISTANCE (kΩ)
0 20 40 60 80 100 120 140
FREQUENCY (kHz)
PHASE (DEG)
800
1200
3501 F02
400
100
1600
600
1000
200
1400
170
180
160
150
190
165
175
155
185
FREQUENCY
PHASE
applicaTions inForMaTion