LTC3786_15 Datasheet, PDF (13/34 Pages) Linear Technology – Low IQ Synchronous Boost Controller

LTC3786
Operation (Refer to the Block Diagram)
The typical capture range of the LTC3786’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).
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 loop keeps 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 4% of the maximum ILIM
current. 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 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 excessive power dissipation
across the body diode of the top MOSFET in this situa-
tion, the chip can be switched over to forced continuous
or pulse-skipping mode to enable the charge pump, or a
Schottky diode can also be placed in parallel to 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).
Operation at Low SENSE Pin Common Mode Voltage
The current comparator in the LTC3786 is powered directly
from the SENSE⁺ pin. This enables the common mode
voltage of SENSE⁺ and SENSE^– pins to operate as low
as 2.5V, which is below the INTV_CC UVLO threshold. The
figure on the first page shows a typical application when
the controller’s VBIAS is powered from V_OUT while V_IN
supply can go as low as 2.5V. If the voltage on SENSE⁺
drops below 2.5V, the SS pin will be held low. When the
SENSE⁺ voltage returns to the normal operating range, the
SS pin will be released, initiating a new soft-start cycle.
BOOST Supply Refresh and Internal Charge Pump
The top MOSFET driver is biased from the floating boot-
strap capacitor, C_B, which normally recharges during each
cycle through an external diode when the bottom MOSFET
turns on. There are two considerations to keep the BOOST
supply at the required bias level. During start-up, if the
bottom MOSFET is not turned on within 100µs after UVLO
goes low, the bottom MOSFET will be forced to turn on
for ~400ns. This forced refresh generates enough BOOST-
SW voltage to allow the top MOSFET to be fully enhanced
instead of waiting for the initial few cycles to charge the
bootstrap capacitor, C_B. There is also an internal charge
pump that keeps the required bias on BOOST. The charge
pump always operates in both forced continuous mode
and pulse-skipping mode. In Burst Mode operation, the
charge pump is turned off during sleep and enabled when
the chip wakes up. The internal charge pump can normally
supply a charging current of 85µA.
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