LTC3600 LINER [Linear Technology], LTC3600 Datasheet - Page 11

• Manufacturer Part Number: LTC3600
• Description: 15V, 1.5A Synchronous Rail-to-Rail Single Resistor
• Manufacturer: LINER [Linear Technology]

OPERATION
R
frequency is within this 30% range of the R
frequency.
Output Voltage Tracking and Soft Start
The LTC3600 allows the user to program its output voltage
ramp rate by means of the ISET pin. Since V
voltage to that of the ISET pin, placing an external capaci-
tor C
the ISET pin and thus the V
The soft-start time t
times of time constant (R
also be driven by an external voltage supply capable of
sinking 50μA.
When starting up into a pre-biased V
stay in discontinuous mode and keep the power switches
off until the voltage on ISET has ramped up to be equal
to V
and V
Output Power Good
When the LTC3600’s output voltage is within the 7.5%
window of the regulation point, which is reflected back
as a V
output voltage is in regulation and the PGOOD pin is
pulled high with an external resistor connected to INTV
or another voltage rail. Otherwise, an internal open-drain
pull-down device (200Ω) will pull the PGOOD pin low.
To prevent unwanted PGOOD glitches during transients
or dynamic V
edge includes a blanking delay of approximately 20μs.
Internal/External ITH Compensation
For ease of use, the user can simplify the loop compen-
sation by tying the ITH pin to INTV
T
value should be selected such that the external clock
V
from 0 to 90% V
t
t
OUT
SS
SS
SET
OUT(t)
OUT
PGFB
, at which point the switcher will begin switching
≅ − R
on the ISET pin will program the ramp-up rate of
2.3R
will ramp up with ISET.
voltage in the range of 0.555V to 0.645V, the
= I
OUT
SET
ISET
SET
changes, the LTC3600’s PGOOD falling
C
R
C
SET
SS
OUT
SET
SET
(from 0% to 90% V
SET
⎡
⎢
⎣
OUT
1− e
n(1− 0.9)
• C
voltage.
−
SET
R
SET
OUT
CC
). The ISET pin can
t
to enable internal
, the LTC3600 will
C
SET
T
OUT
programmed
⎤
⎥
⎦
OUT
servos its
) is 2.3
CC
compensation. This connects an internal 100k resistor
in series with a 50pF capacitor to the output of the error
amplifier (internal ITH compensation point). This is a
trade-off for simplicity instead of OPTI-LOOP
tion, where ITH components are external and are selected
to optimize the loop transient response with minimum
output capacitance.
Minimum Off-Time Considerations
The minimum off-time, t
of time that the LTC3600 is capable of turning on the bot-
tom power MOSFET, tripping the current comparator and
turning the power MOSFET back off. This time is generally
about 50ns. The minimum off-time limit imposes a maxi-
mum duty cycle of t
duty cycle is reached, due to a dropping input voltage
for example, then the output will drop out of regulation.
The minimum input voltage to avoid dropout is:
Conversely, the minimum on-time is the smallest dura-
tion of time in which the top power MOSFET can be in
its “on” state. This time is typically 20ns. In continuous
mode operation, the minimum on-time limit imposes a
minimum duty cycle of:
Where t
shows, reducing the operating frequency will alleviate the
minimum duty cycle constraint.
In the rare cases where the minimum duty cycle is sur-
passed, the output voltage will still remain in regulation, but
the switching frequency will decrease from its programmed
value. This is an acceptable result in many applications, so
this constraint may not be of critical importance in most
cases. High switching frequencies may be used in the
design without any fear of severe consequences. As the
sections on inductor and capacitor selection show, high
switching frequencies allow the use of smaller board com-
ponents, thus reducing the size of the application circuit.
D
V
MIN
IN(MIN)
ON(MIN)
= f
SW
= V
• t
is the minimum on-time. As the equation
OUT
ON(MIN)
ON
•
t
/(t
ON
OFF(MIN)
ON
+ t
+ t
t
ON
OFF(MIN)
OFF(MIN)
, is the smallest amount
). If the maximum
LTC3600
®
optimiza-
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