1766 查看數據表（PDF） - Linear Technology

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1766

5.5V to 60V 1.5A, 200kHz Step-Down Switching Regulator

Linear Technology 

APPLICATIONS INFORMATION

LT1766

TO FREQUENCY

SHIFTING

ERROR

AMPLIFIER

1.4V

1.2V

R3

1k

Q1

R4

2k

BUFFER

Q2

TO SYNC CIRCUIT

LT1766/LT1766-5

VSW

L1

OUTPUT

5V

R1

FB

+

C1

R2

5k

VC GND

1766 F02

Figure 2. Frequency and Current Limit Foldback

divider impedance is not critical, caution should be used if

resistors are increased beyond the suggested values and

short-circuit conditions occur with high input voltage. High

frequency pickup will increase and the protection accorded

by frequency and current foldback will decrease.

40mV/DIV

VOUT AT IOUT = 1A

VOUT AT IOUT = 0.1A

CHOOSING THE INDUCTOR

For most applications, the output inductor will fall into

the range of 15μH to 100μH. Lower values are chosen to

reduce physical size of the inductor. Higher values allow

more output current because they reduce peak current

seen by the LT1766 switch, which has a 1.5A limit. Higher

values also reduce output ripple voltage.

When choosing an inductor you will need to consider

output ripple voltage, maximum load current, peak induc-

tor current and fault current in the inductor. In addition,

other factors such as core and copper losses, allowable

component height, EMI, saturation and cost should also

be considered. The following procedure is suggested

as a way of handling these somewhat complicated and

conflicting requirements.

Output Ripple Voltage

Figure 3 shows a typical output ripple voltage wave-

form for the LT1766. Ripple voltage is determined by

ripple current (ILP-P) through the inductor and the high

frequency impedance of the output capacitor. The fol-

lowing equations will help in choosing the required

0.5A/DIV

INDUCTOR CURRENT

AT IOUT = 1A

VIN = 40V

VOUT = 5V

L = 47μH

2.5μs/DIV

C = 100μF, 10V, 0.1Ω

INDUCTOR CURRENT

AT IOUT = 0.1A

1766 F03

Figure 3. LT1766 Ripple Voltage Waveform

inductor value to achieve a desirable output ripple volt-

age level. If output ripple voltage is of less importance,

the subsequent suggestions in Peak Inductor and Fault

Current and EMI will additionally help in the selection of

the inductor value.

Peak-to-peak output ripple voltage is the sum of a triwave

(created by peak-to-peak ripple current (ILP-P) times ESR)

and a square wave (created by parasitic inductance (ESL)

and ripple current slew rate). Capacitive reactance is as-

sumed to be small compared to ESR or ESL.

VRIPPLE

=

(ILP-P

)(ESR)

+

(ESL)

dI

dt

1766fc

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