11

LTC1553L

Soft Start and Current Limit

The LTC1553L includes a soft start circuit which is used

for initial start-up and during current limit operation. The

SS pin requires an external capacitor to GND with the value

determined by the required soft start time. An internal

11

µA current source is included to charge the external SS

capacitor. During start-up, the COMP pin is clamped to a

diode drop above the voltage at the SS pin. This prevents

the error amplifier, ERR, from forcing the loop to maxi-

mum duty cycle. The LTC1553L will begin to operate at low

amplifier begins to regulate the output. The MIN compara-

tor is disabled when soft start is active to prevent it from

overriding the soft start function.

The LTC1553L includes yet another feedback loop to

control operation in current limit. Just before every falling

edge of G1, the current comparator, CC, samples and

holds the voltage drop measured across the external

measured voltage across Q1 increases due to the drop

ceeded the maximum level, CC starts to pull current out of

the external soft start capacitor, cutting the duty cycle and

controlling the output current level. The CC comparator

pulls current out of the SS pin in proportion to the voltage

conditions, the SS pin will fall gradually, creating a time

delay before current limit takes effect. Very short, mild

overloads may not affect the output voltage at all. More

significant overload conditions will allow the SS pin to

reach a steady state, and the output will remain at a

reduced voltage until the overload is removed. Serious

overloads will generate a large overdrive at CC, allowing it

to pull SS down quickly and preventing damage to the

output components.

the current limiting circuit eliminates an expensive dis-

crete sense resistor that would otherwise be required. This

helps minimize the number of components in the high

current path. Due to switching noise and variation of

accurate. The current limiting circuitry is primarily meant

to prevent damage to the power supply circuitry during

fault conditions. The exact current level where the limiting

circuit begins to take effect will vary from unit to unit as the

For a given current limit level, the external resistor from

R

IR

I

IMAX

LMAX

DS ON Q

IMAX

)

() 1

where,

II

I

LMAX

LOAD

RIPPLE

=+

2

=

−

()( )

()( )( )

VV

V

fL

V

IN

OUT

OUT

OSC

O

IN

Figure 5. Current Limit Setting

Q1

180

µA

G1

Q2

1553L F05

–

+

CC

G2

20

Ω

LTC1553L

8

7

+

+