LTC3899_15 Datasheet, PDF (27/38 Pages) Linear Technology – 60V Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3899
Applications Information
peak current of at least 50mA and must be bypassed to
ground with a minimum of 4.7μF ceramic capacitor. Good
bypassing is needed to supply the high transient currents
required by the MOSFET gate drivers and to prevent in-
teraction between the channels. The INTV_CC supply must
be bypassed with a 0.1μF ceramic capacitor.
The DRVSET pin programs the DRV_CC supply voltage as
well as the DRV_CC, UVLO, and EXTV_CC switchover threshold
voltages. Table 1 summarizes the different DRVSET pin
configurations along with the voltage settings that go with
each configuration. Tying the DRVSET pin to INTV_CC pro-
grams DRV_CC to 10V and chooses the higher UVLO/EXTV_CC
thresholds. Tying the DRVSET pin to GND programs DRV_CC
to 6V and chooses the lower UVLO/EXTV_CC thresholds.
By placing a 50k to 100k resistor between DRVSET and
GND the DRV_CC voltage can be programmed between 5V
to 10V, as shown in Figure 9. With a resistor on DRVSET,
the lower UVLO/EXTV_CC thresholds are chosen.
Table 1
DRVSET PIN
0V
INTV_CC
Resistor to GND
50k to 100k
DRV_CC
VOLTAGE
6V
10V
5V to 10V
DRV_CC UVLO
RISING / FALLING
THRESHOLDS
4.0V / 3.8V
7.5V / 6.7V
4.0V / 3.8V
EXTV_CC
SWITCHOVER
THRESHOLD
4.7V
7.7V
4.7V
11
10
9
8
7
6
5
4
50 55 60 65 70 75 80 85 90 95 100
DRVSET PIN RESISTOR (kΩ)
3899 F10
Figure 9. Relationship Between DRV_CC Voltage
and Resistor Value at DRVSET Pin
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the
maximum junction temperature rating for the LTC3899
to be exceeded. The DRV_CC current, which is dominated
by the gate charge current, may be supplied by either the
V_BIAS LDO or the EXTV_CC LDO. When the voltage on the
EXTV_CC pin is less than its switchover threshold (4.7V or
7.7V as determined by the DRVSET pin described above),
the V_BIAS LDO is enabled. Power dissipation for the IC in
this case is highest and is equal to V_BIAS • I_DRVCC. The
gate charge current is dependent on operating frequency
as discussed in the Efficiency Considerations section.
The junction temperature can be estimated by using the
equations given in Note 2 of the Electrical Characteristics.
For example, using the LTC3899 in the QFN package and
setting DRV_CC to 6V, the DRV_CC current is limited to less
than 45mA from a 40V supply when not using the EXTV_CC
supply at a 70°C ambient temperature:
T_J = 70°C + (45mA)(40V – 6V)(34°C/W) = 125°C
To prevent the maximum junction temperature from be-
ing exceeded, the V_BIAS supply current must be checked
while operating in forced continuous mode (PLLIN/MODE
= INTV_CC) at maximum V_BIAS.
When the voltage applied to EXTV_CC rises above its
switch­over threshold, the V_BIAS LDO is turned off and the
EXTV_CC LDO is enabled. The EXTV_CC LDO remains on as
long as the voltage applied to EXTV_CC remains above the
switchover threshold minus the comparator hysteresis.
The EXTV_CC LDO attempts to regulate the DRV_CC voltage
to the voltage as programmed by the DRVSET pin, so while
EXTV_CC is less than this voltage, the LDO is in dropout
and the DRV_CC voltage is approximately equal to EXTV_CC.
When EXTV_CC is greater than the programmed voltage,
up to an absolute maximum of 14V, DRV_CC is regulated
to the programmed voltage.
Using the EXTV_CC LDO allows the MOSFET driver and
control power to be derived from one of the LTC3899’s
switching regulator outputs (4.7V/7.7V ≤ V_OUT ≤ 14V)
during normal operation and from the V_BIAS LDO when
the output is out of regulation (e.g., start-up, short circuit).
If more current is required through the EXTV_CC LDO than
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