LTC3788_15 Datasheet, PDF (14/32 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788
Operation
The typical input clock thresholds on the PLLIN/MODE
pin are 1.6V (rising) and 1.2V (falling).
PolyPhase Applications (CLKOUT and PHASMD Pins)
The LTC3788 features two pins (CLKOUT and PHASMD)
that allow other controller ICs to be daisychained with the
LTC3788 in PolyPhase^® applications. The clock output
signal on the CLKOUT pin can be used to synchronize
additional power stages in a multiphase power supply
solution feeding a single, high current output or multiple
separate outputs. The PHASMD pin is used to adjust the
phase of the CLKOUT signal as well as the relative phases
between the two internal controllers, as summarized in
Table 1. The phases are calculated relative to the zero
degrees phase being defined as the rising edge of the
bottom gate driver output of controller 1 (BG1).
Table 1.
V_PHASMD
GND
Floating
INTV_CC
CONTROLLER 2
PHASE (°C)
180
180
240
CLKOUT
PHASE (°C)
60
90
120
CLKOUT is disabled when one of the channels is in sleep
mode and another channel is either in shutdown or in
sleep mode.
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
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
14
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 directly
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 during
each cycle through an external diode when the bottom
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