LT1578 Просмотр технического описания (PDF) - Linear Technology

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LT1578

1.5A, 200kHz Step-Down Switching Regulator

Linear Technology 

LT1578/LT1578-2.5

APPLICATIONS INFORMATION

In addition to lower switching frequency, the LT1578 also

operates at lower switch current limit when the feedback

pin voltage drops below 0.7V. Q2 in Figure 2 performs this

function by clamping the VC pin to a voltage less than its

normal 2.1V upper clamp level. This foldback current limit

greatly reduces power dissipation in the IC, diode and

inductor during short-circuit conditions. External synchro-

nization is also disabled to prevent interference with

foldback operation. Again, it is nearly transparent to the

user under normal load conditions. The only loads that may

be affected are current sources, such as lamps and mo-

tors, that maintain high load current with output voltage

less than 50% of final value. In these rare situations the

feedback pin can be clamped above 0.7V to defeat foldback

current limit. Caution: clamping the feedback pin means

that frequency shifting will also be defeated, so a combina-

tion of high input voltage and dead shorted output may

cause the LT1578 to lose control of current limit.

The internal circuitry which forces reduced switching

frequency also causes current to flow out of the feedback

pin when output voltage is low. The equivalent circuitry is

shown in Figure 2. Q1 is completely off during normal

operation. If the FB pin falls below 0.7V, Q1 begins to

conduct current and reduces frequency at the rate of

approximately 1kHz/µA. To ensure adequate frequency

foldback (under worst-case short-circuit conditions), the

external divider Thevinin resistance must be low enough

to pull 35µA out of the FB pin with 0.5V on the pin (RDIV ≤

14.3k). The net result is that reductions in frequency and

current limit are affected by output voltage divider imped-

ance. Although divider impedance is not critical, caution

should be used if resistors are increased beyond the

suggested values and short-circuit conditions will occur

with high input voltage. High frequency pickup will

increase and the protection accorded by frequency and

current foldback will decrease.

MAXIMUM OUTPUT LOAD CURRENT

Maximum load current for a buck converter is limited by

the maximum switch current rating (IP) of the LT1578.

This current rating is 1.5A up to 50% duty cycle (DC),

decreasing to 1.3A at 80% duty cycle. This is shown

graphically in Typical Performance Characteristics and as

shown in the formula below:

IP = 1.5A for DC ≤ 50%

IP = 1.67 – 0.18 (DC) – 0.32(DC)2 for 50% < DC < 90%

DC = Duty cycle = VOUT/VIN

Example: with VOUT = 5V, VIN = 8V; DC = 5/8 = 0.625, and;

ISW(MAX) = 1.67 – 0.18 (0.625) – 0.32(0.625)2 = 1.43A

Current rating decreases with duty cycle because the

LT1578 has internal slope compensation to prevent cur-

rent mode subharmonic switching. For more details, read

Application Note 19. The LT1578 is a little unusual in this

regard because it has nonlinear slope compensation which

gives better compensation with less reduction in current

limit.

Maximum load current would be equal to maximum

switch current for an infinitely large inductor, but with

finite inductor size, maximum load current is reduced by

one-half peak-to-peak inductor current. The following

formula assumes continuous mode operation, implying

that the term on the right is less than one-half of IP.

( )( ) IOUT(MAX) =

( )( )( ) Continuous Mode

IP −

VOUT VIN − VOUT

2 L f VIN

For the conditions above and L = 15µH,

( )( ) 5 8 − 5

( ) ( ) IOUT MAX

= 1.43 −

215

•

10−

6



200

•

103

8

= 1.43 − 0.31= 1.12A

At VIN = 15V, duty cycle is 33%, so IP is just equal to a fixed

1.5A, and IOUT(MAX) is equal to:

(5)(15 − 5)

( ) 1.5 −

2

15

•

10−

6



200

•

103

15

= 1.5 − 0.56 = 0.94A

9

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