9

LTC1504A

phase margin. The typical applications in this data sheet

show compensation values that work with several combi-

nations of external components—use them as a starting

point. For complex cases or stubborn oscillations, contact

the LTC Applications Department.

External Schottky Diode

An external Schottky diode can be included across the

internal N-channel switch (Q2) to improve efficiency at

heavy loads. The diode carries the inductor current during

the nonoverlap time while the LTC1504A turns Q1 off and

Q2 on and prevents current from flowing in the intrinsic

body diode in parallel with Q2. This diode will improve

efficiency by a percentage point or two as output current

approaches 500mA and can help minimize erratic behav-

ior at very high peak current levels caused by excessive

parasitic current flow through Q2. A Motorola MBRS0530L

is usually adequate, with the cathode connected to SW and

the anode connected to GND. Note that this diode is not

required for normal operation and has a negligible effect

on efficiency at low (< 250mA) output currents.

COMP

FB

*ADJUSTABLE PARTS ONLY

1504A • F04a

LTC1504A

COMP

FB

*ADJUSTABLE PARTS ONLY

1504A • F04b

LTC1504A

Figure 4a. Minimum Compensation Network

Figure 4b. Optimum Compensation Network

Soft Start and Current Limit

Soft start and current limit are linked in the LTC1504A. Soft

start works in a straightforward manner. An internal 12

µA

current source connected to the SS pin will pull up an

external capacitor connected from SS to GND at a rate

determined by the capacitor value. COMP is clamped to a

voltage one diode drop above SS; as SS rises, COMP will

rise at the same rate. When COMP reaches roughly 2V

the output comes into regulation. As SS continues to rise,

the feedback amplifier takes over at COMP, the clamp

Current limit operates by pulling down on the soft start pin

when it senses an overload condition at the output. The

across the internal P-channel switch (Q1) during its on

12

µA pull-down, allowing the voltage to be set by a single

current out of the external soft start capacitor, reducing

the voltage at SS. A soft start capacitor should always be

used if current limit is enabled. SS, in turn, pulls down on

COMP, limiting the output duty cycle and controlling the

output current. When the current overload is removed, the

it were completing a soft start cycle. The size of the

external soft start capacitor controls both how fast the

current limit responds once an overload is detected and

how fast the output recovers once the overload is re-

moved. The soft start capacitor also compensates the

shift due to the output inductor and capacitor do not come

into play and the loop can be adequately compensated

with a single capacitor. Usually a 0.1

µF ceramic capacitor

from SS to GND provides adequate soft start behavior and

acceptable current limit response.

This type of current limit circuit works well with mild

current overloads and eliminates the need for an external

current sensing resistor, making it attractive for LTC1504A

applications. These same features also handicap the cur-

rent limit circuit under severe short circuits when the