LTC3784_15 Datasheet, PDF (13/38 Pages) Linear Technology – 60V PolyPhase Synchronous Boost Controller

LTC3784
Operation
the PLL loop only needs to make slight changes to the
VCO input in order to synchronize the rising edge of the
external clock’s to the rising edge of BG1. The ability to
prebias the loop filter allows the PLL to lock-in rapidly
without deviating far from the desired frequency.
The typical capture range of the LTC3784’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.
PolyPhase Applications (CLKOUT and PHASMD Pins)
The LTC3784 features two pins, CLKOUT and PHASMD,
that allow other controller ICs to be daisy-chained with
the LTC3784 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). Depend-
ing on the phase selection, a PolyPhase application with
multiple LTC3784s can be configured for 2-, 3-, 4- , 6- and
12-phase operation.
Table 1.
V_PHASMD
GND
CONTROLLER 2 PHASE (°)
180
CLKOUT PHASE (°)
60
Floating
180
90
INTV_CC
240
120
CLKOUT is disabled when the controller is in shutdown
or in sleep mode.
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).
For more information www.linear.com/LTC3784
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