9

LTC1504

C of the output stage form a 2nd order roll-off with 180

° of

phase shift; the R due to ESR forms a single zero at a

somewhat higher frequency that reduces the roll-off to

first order and reduces the phase shift to 90

°.

If the output capacitor has a relatively high ESR, the zero

comes in well before the initial phase shift gets all the way

to 180

° and the loop only requires a single small capacitor

from COMP to GND to remain stable (Figure 4a). If, on the

other hand, the output capacitor is a low ESR type to

maximize transient response, the ESR zero can increase in

frequency by a decade or more and the output stage phase

shift can get awfully close to 180

° before it turns around

and comes back to 90

°. Large value ceramic, OS-CON

electrolytic and low impedance tantalum capacitors fall

into this category. These loops require an additional zero

to be inserted at the COMP pin; a series RC in parallel with

a smaller C to ground will usually ensure stability. Figure

4b shows a typical compensation network which will

optimize transient response with most output capacitors.

Adjustable output parts can add a feedforward capacitor

across the feedback resistor divider to further improve

phase margin. The typical applications in this data sheet

COMP

FB

*ADJUSTABLE PARTS ONLY

1504 • F04a

LTC1504

COMP

FB

*ADJUSTABLE PARTS ONLY

1504 • F04b

LTC1504

Figure 4a. Minimum Compensation Network

Figure 4b. Optimum Compensation Network

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 LTC1504 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.

Soft Start and Current Limit

Soft start and current limit are linked in the LTC1504. 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

MIN feedback comparator is disabled to prevent it from

overriding the COMP pin and forcing the output to maxi-

mum duty cycle.

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