LTC3788-1 Datasheet, PDF (13/28 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788-1
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 con-
tinuous mode, the 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.
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 thresh-
old current is set approximately to 4% of the maximum
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 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 pin is connected to an open-drain of an in-
ternal N-channel MOSFET. The MOSFET turns on and pulls
the PGOOD1 pin low when the corresponding VFB1 pin
voltage is not within ±10% of the 1.2V reference voltage.
The PGOOD1 pin is also pulled low when the corresponding
RUN1 pin is low (shut down). When the VFB1 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 LTC3788-1 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. Figure 1
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.
37881f
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