LT3501EFE Linear Technology, LT3501EFE Datasheet - Page 11

• Manufacturer Part Number: LT3501EFE
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,BIPOLAR,TSSOP,20PIN,PLASTIC
• Manufacturer: Linear Technology

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 frequecy 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
effi ciency 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.
R
Frequency Hz
1
ON(MIN)
=
R
D
is the forward voltage drop of the catch diode (D1
2
•
SW
⎛
⎝ ⎜
in the minimum on time of the switch, all at
T
V
0 8
( )
is the voltage drop of the internal switch,
/SYNC pin is internally regulated at 0.975V.
OUT
.
V
=
–
V
1
Table 1. Effi ciency 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 defi ned as the highest input voltage that maintains constant output voltage ripple.
+
V
+
D
V
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 tradeoffs 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
16
18
22
24
24
V
Temperature = 0°C to 85°C
t
graph), V
Max Frequency
R SYNC
ON(MIN)
IN
Figure 2. Frequency and Phase vs R
T
†
/
= 25V, V
RT/SYNC
1.5µH
2.2µH
3.3µH
4.7µH
1600
1400
1200
1000
10µH
800
600
400
200
100
= 200ns (85°C from Typical Characteristics
L*
D
~
= 0.6V, V
0
OUT
42
Values. 3.3V Output
20
k Figure
100µF
22µH
47µH
47µF
47µF
= 3.3V, I
FREQUENCY
=
(
C*
40
25 0 4 0 6
SW
RESISTANCE (kΩ)
3 3 0 6
– .
60
.
= 0.4V (85°C)
172mm
OUT
63mm
66mm
66mm
66mm
2
C + L
+
80
)
+
= 2.5A,
.
2
2
2
2
2
100 120 140
.
T
/SYNC Resistance
PHASE
•
200e-9
3501 F02
1
LT3501
190
185
180
175
170
165
160
155
150
~
750
11
3501fb
k k Hz