LTC1439 Linear Technology, LTC1439 Datasheet - Page 12

• Manufacturer Part Number: LTC1439
• Description: Dual High Efficiency/ Low Noise/ Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIONS
The basic LTC1439 application circuit is shown in Fig-
ure 1. External component selection is driven by the load
requirement and begins with the selection of R
R
power MOSFETs and D1 are selected. Finally, C
are selected. The circuit shown in Figure 1 can be config-
ured for operation up to an input voltage of 28V (limited by
the external MOSFETs).
R
R
The LTC1438/LTC1439 current comparator has a maxi-
mum threshold of 150mV/R
mode range of SGND to INTV
threshold sets the peak of the inductor current, yielding a
maximum average output current I
value less half the peak-to-peak ripple current, I
Allowing some margin for variations in the LTC1438/
LTC1439 and external component values yield:
The LTC1438/LTC1439 work well with values of R
from 0.005 to 0.2 .
C
The LTC1438/LTC1439 use a constant frequency archi-
tecture 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
LTC1438/LTC1439
12
OSC
SENSE
SENSE
SENSE
R
C
OSC
SENSE
Selection for Operating Frequency
Selection for Output Current
is known, C
is chosen based on the required output current.
( )
pF
OSC
100
OSC
I
MAX
is charged by a fixed current plus an
Frequency (kHz)
mV
is calculated from the desired operating
U
1 37 10
OSC
. (
PLLLPF
OSC
and L can be chosen. Next, the
OSC
INFORMATION
U
. Each time the topside MOSFET
4
= 0V):
SENSE
is reset to ground. During the
CC
)
. The current comparator
PLLLPF
and an input common
MAX
W
11
)(LTC1439 only).
equal to the peak
SENSE
IN
U
and C
L
. Once
.
OSC
SENSE
OUT
is
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
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 increases with higher V
I
L
( )( )
300
250
200
150
100
f L
50
0
1
0
Figure 2. Timing Capacitor Value
V
OUT
100
OPERATING FREQUENCY (kHz)
1
OSC
–
200
V
L
V
OUT
vs frequency is given in Figure
decreases with higher induc-
IN
300
V
PLLLPF
400
LTC1435 • F02
OSC
= 0V
500
corresponding
IN
or V
OUT
: