LTC3892_15 Datasheet, PDF (23/36 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3892/LTC3892-1
Applications Information
DRVSET pin to GND programs DRV_CC to 6V. 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 8.
Table 2a
DRVSET PIN
GND
INTV_CC
Resistor to GND 50k to 100k
DRV_CC VOLTAGE
6V
10V
5V to 10V
Table 2b
DRVUV PIN
GND
INTV_CC
DRV_CC UVLO
RISING / FALLING
THRESHOLDS
4.0V / 3.8V
7.5V / 6.7V
EXTV_CC SWITCHOVER
RISING / FALLING
THRESHOLD
4.7V / 4.45V
7.7V / 7.45V
11
10
9
8
7
6
5
4
50 55 60 65 70 75 80 85 90 95 100
DRVSET PIN RESISTOR (kΩ)
38921 F09
Figure 8. 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 LTC3892/
LTC3892-1 to be exceeded. The DRV_CC current, which is
dominated by the gate charge current, may be supplied by
either the V_IN 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 DRVUV pin described
above), the V_IN LDO is enabled. Power dissipation for the
IC in this case is highest and is equal to V_IN • 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 LTC3892 in the QFN package and
setting DRV_CC to 6V, the DRV_CC current is limited to less
than 37mA from a 40V supply when not using the EXTV_CC
supply at a 70°C ambient temperature:
T_J = 70°C + (37mA)(40V – 6V)(44°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the V_IN supply current must be checked while
operating in forced continuous mode (PLLIN/MODE =
INTV_CC) at maximum V_IN.
When the voltage applied to EXTV_CC rises above its
switchover threshold, the V_IN 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 LTC3892/
LTC3892-1’s switching regulator outputs (4.7V/7.7V ≤
V_OUT ≤ 14V) during normal operation and from the V_IN 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 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_IN.
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
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