Datasheet

9
LTC1504
APPLICATIONS INFORMATION
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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
C
C
*ADJUSTABLE PARTS ONLY
1504 • F04a
LTC1504
R
FB2
*
R
FB1
*
V
OUT
COMP
FB
C
C
*ADJUSTABLE PARTS ONLY
1504 • F04b
LTC1504
C
F
C
FF
*
R
C
R
FB2
*
R
FB1
*
V
OUT
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 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
. During a soft start cycle, the
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
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