6
DEMO MANUAL DC257
NO-DESIGN SWITCHER
OPERATIO
U
maximum value. As C
SS
continues to charge, I
TH
is gradu-
ally 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.
SYNC/MODE Pin Function (Frequency Synchronization
and Burst Mode Disable)
The LTC1707 can be synchronized with an external
TTL/CMOS-compatible clock signal driving the SYNC/
MODE pin (E8). The frequency range of this signal must be
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 clocked by an external source, Burst
Mode operation is disabled; the LTC1707 then operates in
PWM pulse-skipping mode. In this mode, when the output
load is very low, current comparator I
COMP
remains tripped
for more than one cycle and forces the main switch to stay
off for the same number of cycles. Increasing the output
load slightly allows constant-frequency PWM operation to
resume.
Frequency synchronization is inhibited when the feedback
voltage, V
FB
, is below 0.6V. This prevents the external
clock from interfering with the frequency foldback for
short-circuit protection.
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 PWM
pulse-skipping mode, connect the SYNC/MODE pin to
GND.
When the converter uses Burst Mode operation, the peak
current of the inductor is set to approximately 200mA,
even though the voltage at the I
TH
pin indicates a lower
value. The voltage at the I
TH
pin drops when the inductor’s
average current is greater than the load requirement. As
the I
TH
voltage drops below 0.12V, the BURST comparator
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
µ
A. The load current is now supplied
from the output capacitor. When the output voltage drops,
causing I
TH
to rise above 0.22V, the top 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 of 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 gradually
increase to 350kHz (or the synchronized frequency) when
V
FB
rises above 0.3V.
DROPOUT OPERATION
When the input supply voltage decreases toward the
output voltage, the duty cycle increases toward the maxi-
mum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one
cycle until it reaches 100% duty cycle. The output voltage
will then be determined by the input voltage minus the
voltage drop across the P-channel MOSFET and the inductor.
In Burst Mode operation or pulse skipping mode opera-
tion with the outputs lightly loaded, the LTC1707 passes
through continuous mode as it enters dropout.