LTC1707 LINER [Linear Technology], LTC1707 Datasheet - Page 6

• Manufacturer Part Number: LTC1707
• Description: High Efficiency Monolithic Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

OPERATIO
LTC1707
Main Control Loop
The LTC1707 uses a constant frequency, current mode
step-down architecture. Both the main (P-channel
MOSFET) and synchronous (N-channel MOSFET) switches
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 com-
parator, I
current at which I
the voltage on the I
amplifier EA. The V
section, allows EA to receive an output feedback voltage
from an external resistive 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
tor 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 next cycle.
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 keeping it off
until the fault is removed.
Burst Mode Operation
The LTC1707 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/MODE pin to float or connect it to a logic
high. To disable Burst Mode operation and enable pulse
skipping mode, connect the SYNC/MODE pin to GND. In
this mode, efficiency is lower at light loads, but becomes
comparable to Burst Mode operation when the output load
exceeds 30mA.
6
TH
SS
voltage clamped at approximately 5% of its maximum
reaches 0.7V, the main control loop is enabled with the
COMP
TH
SS
voltage to increase until the average induc-
, resets the RS latch. The peak inductor
continues to charge, I
COMP
U
FB
TH
pin, described in the Pin Functions
(Refer to Functional Diagram)
resets the RS latch is controlled by
pin, which is the output of error
RCMP
, or the beginning of
TH
is gradually
SS
. When
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 forcing
off both internal 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 LTC1707 can be synchronized with an external
TTL/CMOS compatible clock signal with an amplitude of at
least 2V
from 385kHz to 550kHz. Do not attempt to synchronize the
LTC1707 below 385kHz as this may cause abnormal
operation and an undesired frequency spectrum. The top
MOSFET turn-on follows the rising edge of the external
source.
When the LTC1707 is synchronized to an external source,
the LTC1707 operates in PWM pulse skipping mode. In
this mode, when the output load is very low, current
comparator I
and forces the main switch to stay off for the same number
of cycles. Increasing the output load slightly allows con-
stant frequency PWM operation to resume. This mode
exhibits low output ripple as well as low audio noise and
reduced RF interference while providing reasonable low
current efficiency.
FB
rises above 0.3V.
TH
voltage drops below 0.12V, the BURST comparator
P-P
. The frequency range of this signal must be
COMP
remains tripped for more than one cycle
TH
TH
pin drops when the inductor’s
to rise above 0.22V, the top
TH
pin indicates a lower