LTC3899_15 Datasheet, PDF (28/38 Pages) Linear Technology – 60V Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3899
Applications Information
is specified, an external Schottky diode can be added
between the EXTV_CC and DRV_CC pins. In this case, do not
apply more than 10V to the EXTV_CC pin and make sure
that EXTV_CC ≤ V_BIAS.
Significant efficiency and thermal gains can be realized
by powering DRV_CC from the output, since the V_IN cur-
rent resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Efficiency).
For 5V to 14V regulator outputs, this means connecting
the EXTV_CC pin directly to V_OUT. Tying the EXTV_CC pin to
an 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
T_J = 70°C + (45mA)(8.5V – 6V)(34°C/W) = 74°C
However, for 3.3V and other low voltage outputs, additional
circuitry is required to derive DRV_CC power from the output.
The following list summarizes the four possible connec-
tions for EXTV_CC:
1. EXTV_CC grounded. This will cause DRV_CC to be powered
from the internal V_BIAS regulator resulting in increased
power dissipation in the LTC3899 at high input voltages.
2. EXTV_CC connected directly to the output of one of the
buck regulators. This is the normal connection for a 5V
to 14V regulator and provides the highest efficiency.
3. EXTV_CC connected to an external supply. If an external
supply is available in the 5V to 14V range, it may be
used to power EXTV_CC providing it is compatible with
the MOSFET gate drive requirements. Ensure that
EXTV_CC < V_BIAS.
4. EXTV_CC connected to an output-derived boost network
off one of the buck regulators. For 3.3V and other low
voltage regulators, efficiency gains can still be realized
by connecting EXTV_CC to an output-derived voltage that
has been boosted to greater than 4.7V/7.7V.
Topside MOSFET Driver Supply (C_B)
External bootstrap capacitors, C_B, connected to the BOOST
pins supply the gate drive voltage for the topside MOSFET.
The LTC3899 features an internal switch between DRV_CC
and the BOOST pin for each controller. These internal
switches eliminate the need for external bootstrap diodes
between DRV_CC and BOOST. Capacitor C_B in the Functional
Diagram is charged through this internal switch from DRV_CC
when the SW pin is low. When the topside MOSFET is to
be turned on, the driver places the C_B voltage across the
gate-source of the MOSFET. This enhances the top MOS-
FET switch and turns it on. The switch node voltage, SW,
rises to V_IN and the BOOST pin follows. With the topside
MOSFET on, the boost voltage is above the input supply:
V_BOOST = V_IN + V_DRVCC (V_BOOST = V_OUT + V_DRVCC 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).
Fault Conditions: Buck Current Limit and
Current Foldback
The LTC3899 includes current foldback for the buck chan-
nels to help limit load current when the output is shorted
to ground. If the buck output voltage falls below 70% of
its nominal output level, then the maximum sense volt-
age is progressively lowered from 100% to 40% of its
maximum selected value. Under short-circuit conditions
with very low duty cycles, the buck channel will begin
cycle skipping in order to limit the short-circuit current.
In this situation the bottom MOSFET will be dissipating
most of the power but less than in normal operation. The
short-circuit ripple current is determined by the minimum
on-time, t_ON(MIN), of the LTC3899 (≈80ns), the input volt-
age and inductor value:
∆I_L(SC)
=
t_ON(MIN)


V_IN
L


The resulting average short-circuit current is:
I_SC
=
40%
• I_{LIM(MAX )}
−
1
2
∆I_L(SC)
Fault Conditions: Buck Overvoltage Protection
(Crowbar)
The overvoltage crowbar is designed to blow a system
input fuse when the output voltage of one of the buck
regulators rises much higher than nominal levels. The
3899f
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