LTC1538-AUX データシートの表示（PDF） - Linear Technology

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LTC1538-AUX

Dual High Efficiency/ Low Noise/ Synchronous Step-Down Switching Regulators

Linear Technology 

LTC1538-AUX/LTC1539

APPLICATIONS INFORMATION

Allowing some margin for variations in the LTC1538-AUX/

LTC1539 and external component values yield:

RSENSE

=

100mV

IMAX

The LTC1538-AUX/LTC1539 work well with values of

RSENSE from 0.005Ω to 0.2Ω.

COSC Selection for Operating Frequency

The LTC1538-AUX/LTC1539 use a constant frequency

architecture with the frequency determined by an external

oscillator 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)(LTC1539 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):

( ) 1.37 104

COSC(pF) = Frequency (kHz) – 11

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).

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 generally increases with higher VIN

or VOUT:

∆IL

=

1

(f)(L)

VOUT

1–

VOUT

VIN





Accepting larger values of ∆IL allows the use of low

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is ∆IL = 0.4(IMAX). Remember, the

maximum ∆IL occurs at the maximum input voltage.

The inductor value also has an effect on low current

operation. The transition to low current operation begins

300

VPLLLPF = 0V

250

200

150

100

50

0

0

100 200 300 400 500

OPERATING FREQUENCY (kHz)

LTC1538 • F02

Figure 2. Timing Capacitor Value

60

VOUT = 5.0V

50

VOUT = 3.3V

VOUT = 2.5V

40

30

20

10

0

0 50 100 150 200 250 300

OPERATING FREQUENCY (kHz)

1538 F03

Figure 3. Recommended Inductor Values

12

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