Datasheet

LTC3646/LTC3646-1
9
36461fa
For more information www.linear.com/LTC3646
operaTion
The LTC3646 and LTC3646-1 are current mode, monolithic,
synchronous, step-down regulators capable of 40V input
operation, and extremely high step down ratios while main
-
taining constant frequency. (Both will be referred to as the
LTC3646 except as specifically noted.) Part operation is
enabled by raising the voltage of the RUN pin above 1.21V.
Main Control Loop
In normal operation a switching cycle is initiated by a
signal from the inductor valley current comparator (I
CMP
in the Block Diagram). The top power MOSFET is turned
on, and a timer is simultaneously started in the on-time
controller. The on-time controller computes the correct on
time (subject to t
ON(MIN)
) based on the desired switching
frequency f
O
, and step-down ratio V
ON
/V
IN
, according to the
formula shown in the Block Diagram. In a typical applica-
tion, the
V
ON
pin should be connected to the output voltage,
V
OUT
. When the timer expires, the top power MOSFET is
turned off and the bottom power MOSFET is turned on
until the current comparator (I
CMP
) trips, restarting the
timer and initiating the next cycle. The inductor current
is monitored by sensing the voltage drop across the
SW
and
PGND nodes of the bottom power MOSFET. The volt-
age at
the ITH node sets the I
CMP
comparator threshold
corresponding to the inductor valley current. The error
amplifier (EA) adjusts the ITH voltage by comparing an
internal 0.6V reference voltage to the feedback signal,
V
FB
, derived from the output voltage. If, for example, the
load current increases, the output voltage will decrease
relative to the 0.6V reference. The ITH voltage will rise
until the average inductor current increases to match the
load current.
At low load currents, the inductor current can drop to
zero or become negative. If the LTC3646 is configured for
Burst Mode operation, this inductor current condition is
detected by the current reversal comparator (I
REV
) which
then shuts off the bottom power MOSFET and places the
part into a low quiescent current sleep state, resulting in
discontinuous operation and increased efficiency at low
load currents. Both power MOSFETs will remain off with
the part in sleep and the output capacitor supplying the
load current until the ITH voltage rises sufficiently to initiate
another cycle. Discontinuous operation is disabled by tying
the MODE/SYNC pin to ground, placing the LTC3646 into
forced continuous mode
. During forced continuous mode
operation,
synchronous operation occurs regardless of
the output load current, and the inductor current trough
levels are allowed to become negative.
The operating frequency is determined by the value of the
R
RT
resistor, which programs the current for the internal
oscillator. An internal phase-locked loop adjusts the switch-
ing regulator
on-time so that the switching frequency
matches the programmed frequency, subject to t
ON
and
t
OFF
time constraints shown in the Electrical Characteristics
table. Alternatively, the RT pin can be connected to the
INTV
CC
pin which causes the internal oscillator to run at
the default frequency of 2.25MHz.
A clock signal can be applied to the MODE/SYNC pin to
synchronize the switching frequency to an external source.
When operating in this configuration, connect a resistor to
the RT pin with a value corresponding to the applied clock
frequency. With an external clock supplied to the MODE/
SYNC pin, the part will operate in forced continuous mode.
Power Good Status Output
The PGOOD open-drain output will be pulled low if the
regulator output exits the V
PGOOD
window around the
regulation point. This condition is released once regula-
tion within
the specified window is achieved. To
prevent
unwanted PGOOD glitches during transients or dynamic
V
OUT
changes, the LTC3646 PGOOD falling edge includes
a filter time of approximately 70 clock cycles.