LTC1504A
APPLICATIONS INFORMATION
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.
V
OUT
R
FB1
*
LTC1504A FB
COMP
C
C
*ADJUSTABLE PARTS ONLY
1504A • F04a
R
FB2
*
Figure 4a. Minimum Compensation Network
V
OUT
R
FB1
*
LTC1504A FB
COMP
R
FB2
*
C
FF
*
R
C
C
F
C
C
*ADJUSTABLE PARTS ONLY
1504A • F04b
Figure 4b. Optimum Compensation Network
U
W
U
U
Soft Start and Current Limit
Soft start and current limit are linked in the LTC1504A. Soft
start works in a straightforward manner. An internal 12µA
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
below V
CC
, the duty cycle will slowly begin to increase until
the output comes into regulation. As SS continues to rise,
the feedback amplifier takes over at COMP, the clamp
releases and SS rises to V
CC
.
Current limit operates by pulling down on the soft start pin
when it senses an overload condition at the output. The
current limit amplifier (I
LIM
) compares the voltage drop
across the internal P-channel switch (Q1) during its on
time to the voltage at the I
MAX
pin. I
MAX
includes an internal
12µA pull-down, allowing the voltage to be set by a single
resistor between V
CC
and I
MAX
. When the IR drop across
Q1 exceeds the drop across the I
MAX
resistor, I
LIM
pulls
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
I
LIM
amplifier lets go of SS and allows it to rise again as if
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
feedback loop created by the I
LIM
amplifier. Because the
I
LIM
loop is a current feedback loop, the additional phase
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 0.1µF 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
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