LTC3838-2_15 Datasheet, PDF (30/56 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with xternal Reference Voltage and Dual Differential Output Sensing

LTC3838-2
APPLICATIONS INFORMATION
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode at maximum V_IN.
When the voltage applied to the EXTV_CC pin rises above
the switchover voltage (typically 4.6V), the V_IN LDO is
turned off and the EXTV_CC is connected to DRV_CC2 pin with
an internal switch. This switch remains on as long as the
voltage applied to EXTV_CC remains above the hysteresis
(around 200mV) below the switchover voltage. Using
EXTV_CC allows the MOSFET driver and control power
to be derived from the LTC3838-2’s switching regulator
output V_OUT during normal operation and from the LDO
when the output is out of regulation (e.g., start up, short
circuit). If more current is required through the EXTV_CC
than is specified, an external Schottky diode can be added
between the EXTV_CC and DRV_CC pins. Do not apply more
than 6V to the EXTV_CC pin and make sure that EXTV_CC is
less than V_IN.
Significant efficiency and thermal gains can be realized
by powering DRV_CC from the switching converter output,
since the V_IN current resulting from the driver and control
currents will be scaled by a factor of (Duty Cycle)/(Switcher
Efficiency).
Tying the EXTV_CC pin to a 5V supply reduces the junction
temperature in the previous example from 125°C to:
T_J = 70°C + (42mA)(5V)(34°C/W) = 77°C
However, for 3.3V and other low voltage outputs, ad-
ditional circuitry is required to derive DRV_CC power from
the converter output.
The following list summarizes the four possible connec-
tions for EXTV_CC:
1. EXTV_CC left open (or grounded). This will cause INTV_CC
to be powered from the internal 5.3V LDO resulting in an
efficiency penalty of up to 10% at high input voltages.
2. EXTV_CC connected directly to switching converter output
V_OUT is higher than the switchover voltage’s higher limit
(4.8V). This provides the highest efficiency.
3. EXTV_CC connected to an external supply. If a 4.8V or
greater external supply is available, it may be used to
power EXTV_CC providing that the external supply is
sufficient for MOSFET gate drive requirements.
4. EXTV_CC connected to an output-derived boost network.
For 3.3V and other low voltage converters, efficiency
gains can still be realized by connecting EXTV_CC to an
output-derived voltage that has been boosted to greater
than 4.8V.
For applications where the main input power never exceeds
5.3V, tie the DRV_CC1 and DRV_CC2 pins to the V_IN input
through a small resistor, (such as 1Ω to 2Ω) as shown
in Figure 8 to minimize the voltage drop caused by the
gate charge current. This will override the LDO and will
prevent DRV_CC from dropping too low due to the dropout
voltage. Make sure the DRV_CC voltage exceeds the R_DS(ON)
test voltage for the external MOSFET which is typically at
4.5V for logic-level devices.
LTC3838-2
DRV_CC2
DRV_CC1
R_DRVCC
C_DRVCC
V_IN
C_IN
38382 F08
Figure 8. Setup for V_IN ≤ 5.3V
Input Undervoltage Lockout (UVLO)
The LTC3838-2 has two functions that help protect the
controller in case of input undervoltage conditions. An
internal UVLO comparator constantly monitors the INTV_CC
and DRV_CC voltages to ensure that adequate voltages are
present. The comparator enables internal UVLO signal,
which locks out the switching action of both channels, until
the INTV_CC and DR_VCC1,2 pins are all above their respective
UVLO thresholds. The rising threshold (to release UVLO)
30
For more information www.linear.com/3838-2
38382f