LTC3769
13
3769fa
For more information www.linear.com/LTC3769
OPERATION
The typical capture range of the LTC3769’s PLL is from
approximately 55kHz to 1MHz, and is guaranteed to lock
to an external clock source whose frequency is between
75kHz and 850kHz.
The typical input clock thresholds on the PLLIN/MODE
pin are 1.6V (rising) and 1.2V (falling). The recommended
maximum amplitude for low level and minimum amplitude
for high level of external clock are 0V and 2.5V, respectively.
Operation When V
IN
> Regulated V
OUT
When V
IN
rises above the regulated V
OUT
voltage, the boost
controller can behave differently depending on the mode,
inductor current and V
IN
voltage. In forced continuous
mode, the control loop works to keep the top MOSFET on
continuously once V
IN
rises above V
OUT
. The internal charge
pump delivers current to the boost capacitor to maintain
a sufficiently high TG voltage. The amount of current the
charge pump can deliver is characterized by two curves
in the Typical Performance Characteristics section.
In pulse-skipping mode, if V
IN
is between 100% and
110% of the regulated V
OUT
voltage, TG turns on if the
inductor current rises above a certain threshold and turns
off if the inductor current falls below this threshold. This
threshold current is set to approximately 6%, 4% or
3% of the maximum ILIM current when the ILIM pin is
grounded, floating or tied to INTV
CC
, respectively. If the
controller is programmed to Burst Mode operation under
this same V
IN
window, then TG remains off regardless of
the inductor current.
If the OVMODE pin is grounded and V
IN
rises above 110%
of the regulated V
OUT
voltage in any mode, the controller
turns on TG regardless of the inductor current. In Burst
Mode operation, however, the internal charge pump turns
off if the chip is asleep. With the charge pump off, there
would be nothing to prevent the boost capacitor from
discharging, resulting in an insufficient TG voltage needed
to keep the top MOSFET completely on. To prevent exces
-
sive power dissipation across the body diode of the top
MOSFET in this situation, the chip can be switched over
to forced continuous mode to enable the charge pump;
a Schottky diode can also be placed in parallel with the
top MOSFET
.
Power Good
The PGOOD pin is connected to an open drain of an
internal N-channel MOSFET. The MOSFET turns on and
pulls the PGOOD pin low when the VFB pin voltage is not
within ±10% of the 1.2V reference voltage. The PGOOD
pin is also pulled low when the corresponding RUN pin
is low (shut down). When the VFB pin voltage is within
the ±10% requirement, the MOSFET is turned off and the
pin is allowed to be pulled up by an external resistor to a
source of up to 6V (abs max).
Overvoltage Mode Selection
The OVMODE pin is used to select how the LTC3769
operates during an overvoltage event, defined as when
the output feedback voltage (V
FB
) is greater than 110%
of its normal regulated point of 1.2V. It is also used to
determine the light-load mode of operation when the
LTC3769 is synchronized to an external clock through the
PLLIN/MODE pin.
The OVMODE pin is a logic input that should normally be
tied to INTV
CC
or grounded. Alternatively, the pin can be
left floating, which allow a weak internal resistor to pull
it down to ground.
OVMODE = INTV
CC
: An overvoltage event causes the
error amplifier to pull the ITH pin low. In Burst Mode
operation, this causes the LTC3769 to go to sleep and TG
and BG are held off. In pulse-skipping mode, BG is held
off and TG will turn on if the inductor current is positive.
In forced continuous mode, TG (and BG) will switch on
and off as the LTC3769 will regulate the inductor current
to a negative peak value (corresponding to ITH = 0V) to
discharge the output.
When OVMODE is tied to INTV
CC
, the LTC3769 operates
in pulse-skipping mode when synchronized.
In summary, with OVMODE = INTV
CC
, the inductor cur-
rent is not allowed to go negative (reverse from output to
input) except in forced continuous mode, where it does
reverse current but in a controlled manner with a regulated
negative peak current. OVMODE should be tied to INT
V
CC
in applications where the output voltage may sometimes
be above its regulation point (for example, if the output
