LTC1538CG-AUX Linear Technology, LTC1538CG-AUX Datasheet - Page 12

• Manufacturer Part Number: LTC1538CG-AUX
• Description: IC SW REG STP-DN SYNC DUAL28SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1538CG-AUX

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 1.19 ~ 9 V
• Current - Output: 50mA
• Frequency - Switching: 125kHz ~ 240kHz
• Voltage - Input: 3.5 ~ 36 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS
Allowing some margin for variations in the LTC1538-AUX/
LTC1539 and external component values yield:
The LTC1538-AUX/LTC1539 work well with values of
R
C
The LTC1538-AUX/LTC1539 use a constant frequency
architecture with the frequency determined by an external
oscillator capacitor on C
turns on, the voltage on C
on-time, C
additional current which is proportional to the output
voltage of the phase detector (V
When the voltage on the capacitor reaches 1.19V, C
reset to ground. The process then repeats.
The value of C
frequency. Assuming the phase-locked loop has no exter-
nal oscillator input (V
A graph for selecting C
2. As the operating frequency is increased the gate charge
losses will be higher, reducing efficiency (see Efficiency
Considerations). The maximum recommended switching
frequency is 400kHz. When using Figure 2 for
LTC1538-AUX/LTC1539
12
OSC
SENSE
R
C
SENSE
OSC
Selection for Operating Frequency
from 0.005 to 0.2 .
( )
pF
300
250
200
150
100
OSC
50
0
0
100
OSC
Figure 2. Timing Capacitor Value
I
MAX
Frequency kHz
is charged by a fixed current plus an
mV
is calculated from the desired operating
100
OPERATING FREQUENCY (kHz)
1 37 10
U
.
PLLLPF
OSC
OSC
200
OSC
INFORMATION
U
. Each time the topside MOSFET
vs frequency is given in Figure
4
= 0V):
is reset to ground. During the
300
PLLLPF
–
V
PLLLPF
11
W
400
LTC1538 • F02
= 0V
)(LTC1539 only).
500
U
OSC
is
synchronizable applications, choose C
to a frequency approximately 30% below your center
frequency. (See Phase-Locked Loop and Frequency
Sychronization).
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because of
MOSFET gate charge losses. In addition to this basic trade
off, the effect of inductor value on ripple current and low
current operation must also be considered.
The inductor value has a direct effect on ripple current. The
inductor ripple current I
tance or frequency and generally increases with higher V
or V
Accepting larger values of I
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is I
maximum I
The inductor value also has an effect on low current
operation. The transition to low current operation begins
OUT
I
L
:
( )( )
f L
Figure 3. Recommended Inductor Values
60
50
10
40
30
20
1
0
L
0
occurs at the maximum input voltage.
V
OUT
50
OPERATING FREQUENCY (kHz)
1
100
–
V
L
V
OUT
L
150
decreases with higher induc-
IN
= 0.4(I
L
200
allows the use of low
V
V
V
MAX
OUT
OUT
OUT
250
OSC
= 5.0V
= 3.3V
= 2.5V
). Remember, the
1538 F03
corresponding
300
IN