LTC1627 Linear Technology, LTC1627 Datasheet - Page 6

• Manufacturer Part Number: LTC1627
• Description: Monolithic Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

OPERATIO
LTC1627
Main Control Loop
The LTC1627 uses a constant frequency, current mode
step-down architecture. Both the main and synchronous
switches, consisting of top P-channel and bottom
N-channel power MOSFETs, are internal. During normal
operation, the internal top power MOSFET is turned on
each cycle when the oscillator sets the RS latch, and
turned off when the current comparator, I
RS latch. The peak inductor current at which I
the RS latch is controlled by the voltage on the I
which is the output of error amplifier EA. The V
described in the Pin Functions section, allows EA to
receive an output feedback voltage from an external resis-
tive divider. When the load current increases, it causes a
slight decrease in the feedback voltage relative to the 0.8V
reference, which, in turn, causes the I
crease until the average inductor current matches the new
load current. While the top MOSFET is off, the bottom
MOSFET is turned on until either the inductor current
starts to reverse as indicated by the current reversal
comparator I
The main control loop is shut down by pulling the RUN/SS
pin low. Releasing RUN/SS allows an internal 2.25 A
current source to charge soft-start capacitor C
C
I
value. As C
released, allowing normal operation to resume.
Comparator OVDET guards against transient overshoots
> 7.5% by turning the main switch off and turning the
synchronous switch on. With the synchronous switch
turned on, the output is crowbarred. This may cause a
large amount of current to flow from V
is damaged, blowing the system fuse.
Burst Mode Operation
The LTC1627 is capable of Burst Mode operation in which
the internal power MOSFETs operate intermittently based
on load demand. To enable Burst Mode operation, simply
allow the SYNC/FCB pin to float or connect it to a logic
high. To disable Burst Mode operation and enable forced
continuous mode, connect the SYNC/FCB pin to GND. In
this mode, the efficiency is lowest at light loads, but
becomes comparable to Burst Mode operation when the
6
TH
SS
voltage clamped at approximately 5% of its maximum
reaches 0.7V, the main control loop is enabled with the
SS
RCMP
continues to charge, I
U
, or the beginning of the next cycle.
(Refer to Functional Diagram)
IN
if the main switch
TH
TH
COMP
voltage to in-
is gradually
COMP
, resets the
SS
. When
TH
FB
resets
pin,
pin,
output load exceeds 100mA. The threshold voltage be-
tween Burst Mode operation and forced continuous mode
is 0.8V. This can be used to assist in secondary winding
regulation as described in Auxiliary Winding Control Using
SYNC/FCB Pin in the Applications Information section.
When the converter is in Burst Mode operation, the peak
current of the inductor is set to approximately 200mA,
even though the voltage at the I
value. The voltage at the I
average current is greater than the load requirement. As
the I
trips, causing the internal sleep line to go high and turn off
both power MOSFETs.
In sleep mode, both power MOSFETs are held off and the
internal circuitry is partially turned off, reducing the quies-
cent current to 200 A. The load current is now being
supplied from the output capacitor. When the output
voltage drops, causing I
MOSFET is again turned on and this process repeats.
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator is reduced to about 35kHz, 1/10 the nominal
frequency. This frequency foldback ensures that the
inductor current has more time to decay, thereby prevent-
ing runaway. The oscillator’s frequency will progressively
increase to 350kHz (or the synchronized frequency) when
V
Frequency Synchronization
The LTC1627 can be synchronized with an external
TTL/CMOS compatible clock signal. The frequency range
of this signal must be from 385kHz to 525kHz. Do not
attempt to synchronize the LTC1627 below 385kHz as this
may cause abnormal operation and an undesired fre-
quency spectrum. The top MOSFET turn-on follows the
rising edge of the external source.
When the LTC1627 is clocked by an external source, Burst
Mode operation is disabled; the LTC1627 then operates in
PWM pulse skipping mode. In this mode, when the output
load is very low, current comparator I
for more than one cycle and forces the main switch to stay
off for the same number of cycles. Increasing the output
FB
rises above 0.3V.
TH
voltage drops below 0.12V, the BURST comparator
TH
TH
pin drops when the inductor’s
to rise above 0.22V, the top
TH
pin indicates a lower
COMP
remains tripped