Datasheet

LTC3859A
29
3859af
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 MOSFETs.
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).
Figure 10. Relationship Between Oscillator
Frequency and Resistor Value at the FREQ Pin
FREQ PIN RESISTOR (kΩ)
15
FREQUENCY (kHz)
600
800
1000
35 45 5525
3859A F10
400
200
500
700
900
300
100
0
65 75 85 95 105 115
125