LTC3859AL_15 Datasheet, PDF (29/44 Pages) Linear Technology – Triple Output, Buck/Buck/Boost Synchronous Controller with 28A Burst Mode IQ

LTC3859AL
applications information
Topside MOSFET Driver Supply (C_B, D_B)
External bootstrap capacitors C_B connected to the BOOST
pins supply the gate drive voltages for the topside MOS-
FETs. Capacitor C_B in the Functional Diagram is charged
though external diode D_B from INTV_CC when the SW pin
is low. When one of the topside MOSFETs is to be turned
on, the driver places the C_B voltage across the gate-source
of the desired MOSFET. This enhances the MOSFET and
turns on the topside switch. The switch node voltage,
SW, rises to V_IN for the buck channels (V_OUT for the boost
channel) and the BOOST pin follows. With the topside
MOSFET on, the boost voltage is above the input supply:
V_BOOST = V_IN + V_INTVCC (V_BOOST = V_OUT + V_INTVCC for the
boost controller). The value of the boost capacitor C_B
needs to be 100 times that of the total input capacitance
of the topside MOSFET(s). The reverse breakdown of the
external Schottky diode must be greater than V_IN(MAX) for
the buck channels and V_OUT(MAX) for the boost channel.
The external diode D_B can be a Schottky diode or silicon
diode, but in either case it should have low leakage and fast
recovery. Pay close attention to the reverse leakage at high
temperatures where it generally increases substantially.
The topside MOSFET driver for the boost channel includes
an internal charge pump that delivers current to the
bootstrap capacitor from the BOOST3 pin. This charge
current maintains the bias voltage required to keep the
top MOSFET on continuously during dropout/overvolt-
age conditions. The Schottky/silicon diode selected for
the boost topside driver should have a reverse leakage
less than the available output current the charge pump
can supply. Curves displaying the available charge pump
current under different operating conditions can be found
in the Typical Performance Characteristics section.
A leaky diode D_B in the boost converter can not only
prevent the top MOSFET from fully turning on but it can
also completely discharge the bootstrap capacitor C_B and
create a current path from the input voltage to the BOOST3
pin to INTV_CC. This can cause INTV_CC to rise if the diode
leakage exceeds the current consumption on INTV_CC. This
is particularly a concern in Burst Mode operation where
the load on INTV_CC can be very small. There is an internal
voltage clamp on INTV_CC that prevents the INTV_CC voltage
from running away, but this clamp should be regarded
as a failsafe only. The external Schottky or silicon diode
should be carefully chosen such that INTV_CC never gets
charged up much higher than its normal regulation voltage.
Care should also be taken when choosing the external
diode D_B for the buck converters. A leaky diode not only
increases the quiescent current of the buck converter, but
it can also cause a similar leakage path to INTV_CC from
V_OUT for applications with output voltages greater than
the INTV_CC voltage (~5.4V).
1000
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0
15 25 35 45 55 65 75 85 95 105 115 125
FREQ PIN RESISTOR (kΩ)
3859al F10
Figure 10. Relationship Between Oscillator
Frequency and Resistor Value at the FREQ Pin
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3859alf
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