LTC3899_15 Datasheet, PDF (17/38 Pages) Linear Technology – 60V Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3899
Operation (Refer to the Functional Diagrams)
Frequency Selection and Phase-Locked Loop
(FREQ and PLLIN/MODE Pins)
The selection of switching frequency is a trade-off between
efficiency and component size. Low frequency opera-
tion increases efficiency by reducing MOSFET switching
losses, but requires larger inductance and/or capacitance
to maintain low output ripple voltage.
The switching frequency of the LTC3899’s controllers can
be selected using the FREQ pin.
If the PLLIN/MODE pin is not being driven by an external
clock source, the FREQ pin can be tied to GND, tied to
INTV_CC or programmed through an external resistor. Tying
FREQ to GND selects 350kHz while tying FREQ to INTV_CC
selects 535kHz. Placing a resistor between FREQ and GND
allows the frequency to be programmed between 50kHz
and 900kHz, as shown in Figure 10.
A phase-locked loop (PLL) is available on the LTC3899
to synchronize the internal oscillator to an external clock
source that is connected to the PLLIN/MODE pin. The
LTC3899’s phase detector adjusts the voltage (through an
internal lowpass filter) of the VCO input to align the turn-
on of controller 1’s external top MOSFET (and controller
3’s external bottom MOSFET) to the rising edge of the
synchronizing signal. Thus, the turn-on of controller 2’s
external top MOSFET is 180° out of phase to the rising
edge of the external clock source.
The VCO input voltage is prebiased to the operating fre-
quency set by the FREQ pin before the external clock is
applied. If prebiased near the external clock frequency,
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 TG1. 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 LTC3899’s phase-locked
loop is from approximately 55kHz to 1MHz, with a guaran-
tee to be between 75kHz and 850kHz. In other words, the
LTC3899’s PLL 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 1.1V (falling). It is recommended
that the external clock source swings from ground (0V)
to at least 2.5V.
Boost Controller Operation When V_IN > V_OUT
When the input voltage to the boost channel rises above
its regulated V_OUT voltage, the controller can behave dif-
ferently depending on the mode, inductor current and
V_IN voltage. In forced continuous mode, the loop works
to keep the top MOSFET on continuously once V_IN rises
above V_OUT. An internal charge pump delivers current to
the boost capacitor from the BOOST3 pin to maintain a
sufficiently high TG voltage. Because the LTC3899 uses
internal switches and does not require external bootstrap
diodes, the charge pump only has to overcome small
leakage currents (board leakage, etc.).
In pulse-skipping mode, if V_IN is between 0% and 10%
above the regulated V_OUT voltage, TG3 turns on if the
inductor current rises above approximately 3% of the
programmed I_LIM current. If the part is programmed in
Burst Mode operation under this same V_IN window, then
TG3 turns on at the same threshold current as long as
the chip is awake (one of the buck channels is awake and
switching). If both buck channels are asleep or shut down
in this V_IN window, then TG3 will remain off regardless of
the inductor current.
If V_IN rises more than 10% above the regulated V_OUT
voltage in any mode, the controller turns on TG3 regard-
less of the inductor current. In Burst Mode operation,
however, the internal charge pump turns off if the entire
chip is asleep (if the two buck channels are also 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. The charge pump turns back
on when the chip wakes up, and it remains on as long as
one of the buck channels is actively switching.
For more information www.linear.com/LTC3899
3899f
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