LTC1438CG-ADJ View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1438CG-ADJ
Linear Technology
'LTC1438CG-ADJ' PDF : 32 Pages View PDF
LTC1438/LTC1439
APPLICATIONS INFORMATION
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 RSENSE. Once
RSENSE is known, COSC and L can be chosen. Next, the
power MOSFETs and D1 are selected. Finally, CIN and COUT
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).
RSENSE Selection for Output Current
RSENSE is chosen based on the required output current.
The LTC1438/LTC1439 current comparator has a maxi-
mum threshold of 150mV/RSENSE and an input common
mode range of SGND to INTVCC. The current comparator
threshold sets the peak of the inductor current, yielding a
maximum average output current IMAX equal to the peak
value less half the peak-to-peak ripple current, ∆IL.
Allowing some margin for variations in the LTC1438/
LTC1439 and external component values yield:
RSENSE
=
100mV
IMAX
The LTC1438/LTC1439 work well with values of RSENSE
from 0.005Ω to 0.2Ω.
COSC Selection for Operating Frequency
The LTC1438/LTC1439 use a constant frequency architec-
ture with the frequency determined by an external oscilla-
tor capacitor on COSC. Each time the topside MOSFET
turns on, the voltage on COSC is reset to ground. During the
on-time, COSC is charged by a fixed current plus an
additional current which is proportional to the output
voltage of the phase detector (VPLLLPF)(LTC1439 only).
When the voltage on the capacitor reaches 1.19V, COSC is
reset to ground. The process then repeats.
The value of COSC is calculated from the desired operating
frequency. Assuming the phase-locked loop has no exter-
nal oscillator input (VPLLLPF = 0V):
COSC
(pF)
=
⎡
⎢
1.37(104 )
⎤
⎥
⎣⎢Frequency (kHz)⎦⎥
−
11
12
A graph for selecting COSC vs frequency is given in Figure
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 COSC corresponding
to a frequency approximately 30% below your center
frequency. (See Phase-Locked Loop and Frequency
Sychronization).
300
VPLLLPF = 0V
250
200
150
100
50
0
0
100 200 300 400 500
OPERATING FREQUENCY (kHz)
LTC1435 • F02
Figure 2. Timing Capacitor Value
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 ∆IL decreases with higher induc-
tance or frequency and increases with higher VIN or VOUT:
∆IL
=
1
(f)(L)
VOUT
⎛⎝⎜1–
VOUT
VIN
⎞
⎠⎟
14389fb
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