LTC1530(RevA) 데이터 시트보기 (PDF) - Linear Technology

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LTC1530
(Rev.:RevA)

High Power Synchronous Switching Regulator Controller

Linear Technology 

LTC1530

APPLICATIO S I FOR ATIO

Figure 5b plots the minimum required RIMAX resistor (kΩ)

versus the maximum operating load current (ILMAX =

ILOAD + IRIPPLE/2) as a function of Q1’s RDS(ON). Note that

during an intial power-up sequence (VOUT = 0V), the

inductor’s start-up current IST is much higher than the

steady-state condition, ILMAX. The difference between IST

and ILMAX is affected by the input power supply slew rate,

the input and output voltages, the LTC1530 soft-start slew

rate, the maximum duty cycle and the inductor and output

capacitor values.

For a given application, the input and output requirements

are known and determine the main inductor and output

capacitor values. These values establish the transient load

recovery time. In general, a low value inductor combined

with high value output capacitance has a short transient

load recovery time at the expense of higher inductor ripple

and start-up current (IRIPPLE and IST). However, if a small

inductor and large value output capacitors are chosen, the

value of RIMAX obtained from Figure 5b may be too small

to allow proper regulator start-up.

During start-up, if IST is higher than the current limit

threshold set by the RIMAX resistor, the LTC1530 current

limit comparator turns on. This comparator then limits

input charging current by reducing duty cycle. During this

time, if VOUT doesn’t increase above one-half of the rated

value, the LTC1530 hard current limit circuit turns on. This

circuit forces the LTC1530 to repeat a soft-start cycle and

the power supply fails to start. If VOUT increases above

one-half of the rated value, the power supply output may

start-up properly depending on whether the limited input

current charges the output capacitor and prevents hard

current limit action.

Therefore, select RIMAX with the start-up current (IST) in

mind. Choosing RIMAX to set the current comparator

threshold above IST ensures proper power supply start-up

as well as recovery from an output fault condition.

Figures 6a and 6b plot the start-up IST vs output capaci-

tance and inductance for unloaded and loaded conditions

with the current limit circuit disabled. Figures 6a and 6b

are provided as examples. Actual IST under start-up con-

ditions must be measured for any application circuit so

that RIMAX can be properly chosen.

10

5500

4500

3500

RIMAX ≥ 500Ω

ILMAX = ILOAD + IRIPPLE/2

Q1 RDS(ON) = 0.05Ω

0.04Ω

0.03Ω

2500

1500

0.02Ω

0.01Ω

500

0 2 4 6 8 10 12 14 16 18 20

ILMAX (A)

1530 F05b

Figure 5b. Minimum Required RIMAX vs ILMAX

25

TA = 25°C

VIN = 5V

20 ILOAD = 0A

15

L = 1.2µH

10

L = 4.7µH

5

L = 2.4µH

0

024

6

8 10 12

OUTPUT CAPACITANCE (mF)

1530 F06a

Figure 6a. Start-Up IST vs Output Capacitance

30

TA = 25°C

25

VIN = 5V

ILOAD = 10A

20

15

10

L = 2.4µH

L = 1.2µH

L = 4.7µH

5

0

02

4

6

8

10 12

OUTPUT CAPACITANCE (mF)

1530 F06b

Figure 6b. Start-Up IST vs Output Capacitance

1530fa

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