LTC1504A
APPLICATIONS INFORMATION
output bypass capacitors and a compensation network
complete the control loop. The LTC1504A adjustable
output parts require an additional pair of resistors to set
the output voltage. The LTC1504A-3.3 parts include an
onboard resistor divider preset to a 3.3V output voltage. A
functional 3.3V output regulator can be constructed with
an LTC1504A-3.3 and as few as four external components.
The LTC1504A feedback loop includes a precision refer-
ence trimmed to 1% (V
REF
), a wide bandwidth
transconductance feedback amplifier (FB) and an onboard
PWM generator (SAW and PWM). The PWM generator is
capable of generating pulse widths from 0% to 100%,
minimizing dropout and maximizing transient response.
The internal sawtooth oscillator typically runs at 200kHz.
Q1 and Q2 are capable of carrying peak currents in excess
of 500mA, with the continuous output power level limited
primarily by the thermal dissipation of the SO-8 package.
With a 5V input and a 3.3V output, the LTC1504A can
supply 500mA of continuous output current with an
appropriate layout. An on-chip current limit circuit, set
with a single external resistor, can be used to help limit
power dissipation. See the Thermal Considerations sec-
tion for more information.
Theory of Operation
The LTC1504A primary feedback loop consists of the main
error amplifier FB, the PWM generator, the output drive
logic and the power switches. The loop is closed with the
external inductor and the output bypass capacitor. The
feedback amplifier senses the output voltage directly at the
SENSE pin for fixed output versions or through an external
resistor divider in the adjustable output version. This
feedback voltage is compared to the 1.265V internal
reference voltage by FB and an error signal is generated at
the COMP pin. COMP is a high impedance node that is
brought out to an external pin for optimizing the loop
compensation.
COMP is compared to a 200kHz sawtooth wave by com-
parator PWM. The output stage takes the PWM signal and
generates nonoverlapping drive for the onboard P- and N-
channel power MOSFETs, which drive the SW pin with a
low impedance image of the PWM waveform. Typical
open-loop output impedance at SW is between 1Ω and
3Ω, depending on supply voltage. This high power pulse
train is filtered by the external inductor and capacitor,
providing a steady DC value at the output node. This node
returns to FB or SENSE, closing the loop.
The LTC1504A includes a second feedback loop that
controls operation in current limit. The I
LIM
amplifier
monitors the voltage at the SW pin while Q1 is on. It
compares this voltage to the voltage at the I
MAX
pin. As the
peak current through Q1 rises, the voltage drop across it
due to its R
ON
increases proportionally. When SW drops
below I
MAX
, indicating the current through Q1 has in-
creased beyond the desired value, I
LIM
starts pulling a
controlled amount of current out of SS, the external soft
start pin. As SS falls, it pulls COMP down with it, limiting
the duty cycle and reducing the output voltage to control
the current. The speed at which the current limit circuit
reacts is set by the value of the external soft start capacitor.
EXTERNAL COMPONENT SELECTION
External components required by the LTC1504A fall into
three categories: input bypass, output filtering and com-
pensation. Additional components to set up soft start and
current limit are usually included as well. A minimum
LTC1504A circuit can be constructed with as few as four
external components; a circuit that utilizes all of the
LTC1504A’s functionality usually includes eight or nine
external components, with two additional feedback resis-
tors required for adjustable parts. See the Typical Applica-
tions section for examples of external component hookup.
Input Bypass
The input bypass capacitor is critical to proper LTC1504A
operation. The LTC1504A includes a precision reference
and a pair of high power switches feeding from the same
V
CC
pin. If V
CC
does not have adequate bypassing, the
switch pulses introduce enough ripple at V
CC
to corrupt
the reference voltage and the LTC1504A will not regulate
accurately. Symptoms of inadequate bypassing include
poor load regulation and/or erratic waveforms at the SW
pin. If an oscilloscope won’t trigger cleanly when looking
at the SW pin, the LTC1504A doesn’t have adequate input
bypass.
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