LTC3788 Datasheet, PDF (15/32 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788
OPERATION
Operation When V_IN > 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 works to keep the top MOSFET on con-
tinuously once V_IN rises above V_OUT. The internal charge
pump delivers current to the boost capacitor to maintain
a sufficiently high TG voltage.
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 I_LIM 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 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 entire chip is asleep
(the other channel is asleep or shut down). 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 situation, the chip can be
switched over to forced continuous mode to enable the
charge pump, or a Schottky diode can also be placed in
parallel to the top MOSFET.
Power Good
The PGOOD1, 2 pin is connected to an open-drain of an
internal N-channel MOSFET. The MOSFET turns on and
pulls the PGOOD1, 2 pin low when the corresponding
VFB1, 2 pin voltage is not within ±10% of the 1.2V refer-
ence voltage. The PGOOD1, 2 pin is also pulled low when
the corresponding RUN1, 2 pin is low (shut down). When
the VFB1, 2 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.
Operation at Low SENSE Pin Common Voltage
The current comparator in the LTC3788 is powered di-
rectly from the SENSE⁺ pin. This enables the common
mode voltage of SENSE⁺ and SENSE^– pins to operate at as
low as 2.5V, which is below the 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
Each top MOSFET driver is biased from the floating
bootstrap capacitor C_B, which normally recharges dur-
ing each cycle through an external diode when the bot-
tom 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 gener-
ates enough BOOST-SW voltage to allow the top MOSFET
ready to be fully enhanced instead of waiting for the initial
few cycles to charge up. 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 55μA.
3788f
15