LTC3103
13
3103fa
For more information www.linear.com/LTC3103
Input Capacitor Selection
The V
IN
pin provides current to the power stages of the
buck converter. It is recommended that a low ESR ceramic
capacitor with a value of at least 10µF be used to bypass
the pin. These capacitors should be placed as close to
the pin as possible and should have a short return path
to the GND pin.
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
V
OUT
=0.6V • 1+
R1
The external divider is connected to the output as shown
in Figure 1. Note that FB divider current is not included in
the LTC3103 quiescent current specification. For improved
transient response, a feedforward capacitor, C
FF
, may be
placed in parallel with resistor R2. The capacitor modifies
the loop dynamics by adding a pole-zero pair to the loop
dynamics which generates a phase boost that can improve
the phase margin and increase the speed of the transient
response, resulting in smaller voltage deviation on load
transients. The zero frequency depends not only on the
value of the feed forward capacitor, but also on the upper
resistor divider resistor. Specifically, the zero frequency,
f
ZERO
, is given by the following equation:
f
ZERO
=
2•π •R2•C
For R2 resistor values of ~1M a 12pF ceramic capacitor
will suffice, however that value may be increased or de-
creased to optimize the converter’s response for a given
set of application parameters. In a Burst Mode application
for instance, a C
FF
= 27pF will lower output voltage ripple
at light load.
Minimum Off-Time/On-Time Considerations
The maximum duty cycle is limited in the LTC3103 by the
boost capacitor refresh time, the rise/fall times of the switch
as well as propagation delays in the PWM comparator, the
level shifts and the gate drive. This minimum off time is
typically 65ns which imposes a maximum duty cycle of:
DC
MAX
= 1 – (f • t
OFF(MIN)
)
where f is the 1.2MHz switching frequency and t
OFF(MIN)
is the minimum off-time. If the maximum duty cycle is
surpassed, due to a dropping input voltage for example,
the output will drop out of regulation. The minimum input
voltage to avoid this dropout condition is:
V
IN(MIN)
=
OUT
1– f • t
OFF(MIN)
( )
Conversely, the minimum on-time is the smallest duration
of time in which the buck switch can be in its “on” state.
This time is limited by similar factors and is typically 70ns.
In forced continuous operation, the minimum on-time limit
imposes a minimum duty cycle of:
DC
MIN
= f • t
ON(MIN)
where t
ON(MIN)
is the minimum on-time. In extreme step-
down ratios where the minimum duty cycle is surpassed,
the output voltage will still be in regulation but the rectifier
switch will remain on for more than one cycle and sub-
harmonic switching will occur to provide a higher effective
duty cycle. The result is higher output voltage ripple. This
is an acceptable result in many applications so this con-
straint may not be of critical importance in some cases.
Precise Undervoltage Lockout
The LTC3103 is in shutdown when the RUN pin is low and
active when the pin is higher than the RUN pin threshold.
The rising threshold of the RUN pin comparator is an ac-
curate 0.8V, with 60mV of hysteresis. This threshold is
enabled when V
IN
is above the 2.5V minimum value. If V
IN
is lower than 2.5V, an internal undervoltage monitor puts
the part in shutdown independent of the RUN pin state.
The RUN pin can be configured as a precise undervoltage
lockout (UVLO) on the V
IN
supply with a resistive divider
FB
R2
R1
C
FF
3103 F01
V
OUT
GND
LTC3103
Figure 1. Setting the Output Voltage
APPLICATIONS INFORMATION
