7
DEMO MANUAL DC195
NO-DESIGN SWITCHER
OPERATIO
U
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 200µA. 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 operation
(externally synchronized) with the outputs lightly loaded,
the LTC1627 transistions through continuous mode as it
enters dropout.
UNDERVOLTAGE LOCKOUT
A precision undervoltage lockout shuts down the LTC1627
when V
IN
drops below 2.5V, making it ideal for single
lithium-ion battery applications. In shutdown, the LTC1627
draws only several microamperes, which is low enough to
prevent deep discharge and possible damage to a lithium-
ion battery nearing its end of charge. A 150mV hysteresis
ensures reliable operation with noisy supplies.
LOW SUPPLY OPERATION
The LTC1627 is designed to operate down to 2.65V supply
voltage. At this voltage the converter is most likely to be
running at high duty cycles or in dropout, where the main
switch is on continuously. Hence, the I
2
R loss is due mainly
to the R
DS(ON)
of the P-channel MOSFET. See the LTC1627
data sheet for additional information.
When V
IN
is low (< 4.5V), the R
DS(ON)
of the P-channel
MOSFET can be lowered by driving its gate below ground.
The top P-channel MOSFET driver makes use of a floating
return pin, V
DR
, to allow biasing below GND. A simple charge
pump bootstrapped to the SW pin realizes a negative
voltage at the V
DR
pin, as shown in Figure 4. This charge
pump can be enabled via jumper JP1 for V
IN
<4.5V. For
V
IN
≥ 4.5V, do not enable the charge pump to ensure that
V
IN
– V
DR
does not exceed its absolute maximum voltage.
A 10V Zener clamp (Z3) prevents V
IN
– V
DR
from exceed-
ing 10V even if V
IN
≥ 5V is inadvertently applied.
When V
IN
decreases to a voltage close to V
OUT
, the loop
may enter dropout and attempt to turn on the P-channel
MOSFET continuously. When the V
DR
charge pump is
enabled, a dropout detector counts the number of oscilla-
tor cycles that the P-channel MOSFET remains on and
periodically forces a brief off period to allow C1 to
recharge. 100% duty cycle is allowed when V
DR
is
grounded.
SLOPE COMPENSATION AND PEAK INDUCTOR
CURRENT
Slope compensation provides stability by preventing
subharmonic oscillations. It works by internally adding a
ramp to the inductor current signal at duty cycles in excess
of 40%. As a result, the maximum inductor peak current
is lower for V
OUT
/V
IN
> 0.4 than when V
OUT
/V
IN
< 0.4. See
the maximum inductor peak current vs duty cycle graph in
Figure 5.
