LTC3811_15 Datasheet, PDF (28/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
APPLICATIONS INFORMATION
The total power dissipation is the sum of these two and
the junction temperature can then be estimated using the
following equation:
T_J = T_A + (P_VCC + P_LDO) • R_θJA
As an example, consider a 2-phase, single-output applica-
tion with a 12V input voltage and a 1.2V output at up to
30A (15A/phase), using the QFN version of the LTC3811.
The upper power MOSFETs are the Renesas RJK0305DPB
(one per phase) and the lower power MOSFETs are the
RJK0301DPB (one per phase). The upper MOSFETs have
a typical R_DS(ON) = 10mΩ at V_GS = 4.5V and a typical Q_G =
8nC. The lower MOSFETs have a typical R_DS(ON) = 3mΩ at
V_GS = 4.5V and a typical Q_G = 32nC. The total gate charge
is therefore 80nC and the operating frequency is 500kHz.
With a maximum ambient temperature of 70°C and a
thermal resistance of 34°C/W for the QFN package,
I_DRVCC = 10mA + 500kHz • 80nC = 50mA
P_DRVCC = 6V • (10mA + 500kHz • 80nC) = 300mW
P_LDO = (12V – 6V) • (10mA + 500kHz • 80nC)
= 300mW
T_J = 70°C + (0.3 + 0.3) • 34°C/W = 90°C
A 20°C rise in the junction temperature and a maximum LDO
current of 50mA are acceptable numbers but could be im-
proved upon by using the EXTV_CC pin to supply power to the
gate drivers. The use of an auxiliary supply connected to the
EXTV_CC pin would reduce the junction temperature rise by
a factor of 2, resulting in a max junction temperature of:
T_J = 70°C + 0.3 • 34°C/W = 80°C
For applications where the internal LDO is being used to
supply power to the IC, to prevent the maximum junction
temperature from being exceeded the input supply cur-
rent should be monitored at maximum V_IN in continuous
conduction mode (i.e., with MODE/SYNC connected to
INTV_CC).
Using the EXTV_CC Pin to Supply Power to the LTC3811
The LTC3811 contains an internal P-channel MOSFET
switch connected between the EXTV_CC and DRV_CC pins.
When the voltage applied to EXTV_CC exceeds 4.5V, the
internal LDO is turned off and the PMOS switch turns on,
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connecting the EXTV_CC pin to the DRV_CC pin and thereby
supplying the internal analog and digital circuitry and
MOSFET gate drive power. Do not apply greater than 7V
to the EXTV_CC pin (its absolute maximum rating) and en-
sure that EXTV_CC < V_IN + 0.3V when using the application
circuits shown. If an external voltage source is applied to
the EXTV_CC pin when the V_IN supply is not present, a diode
can be placed in series with the LTC3811’s V_IN pin and a
Schottky diode between the EXTV_CC pin and the V_IN pin,
to prevent current from backfeeding into V_IN through the
PMOS body diodes.
Significant energy gains can be realized by power-
ing DRV_CC and INTV_CC from an auxiliary supply, since
the V_IN current resulting from the driver and analog
control circuitry currents will be scaled by the ratio:
Duty Cycle/Efficiency
The following list summarizes the three possible connec-
tions for EXTV_CC:
1. EXTV_CC left open (or grounded). This will cause DRV_CC
and INTV_CC to be powered from the internal 6V LDO,
resulting in a significant efficiency penalty and excess
power dissipation at high input voltages.
2. EXTV_CC connected to an external supply. If an external
supply is available in the 5V to 7V range it may be used
to power EXTV_CC, provided it is capable of satisfying
the gate drive and control IC current requirements. V_IN
must be greater than or equal to the voltage applied to
the EXTV_CC pin.
3. EXTV_CC connected to an output-derived boost network.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV_CC to
an output-derived voltage which has been boosted to
greater than 4.5V but less than 7V. This can be done
with a capacitive charge pump shown in Figure 16.
Power MOSFET and Schottky Diode (Optional)
Selection
Two external power MOSFETs must be selected for each
controller in the LTC3811: one N-channel MOSFET for the
top (main) switch, and one N-channel MOSFET for the
bottom (synchronous) switch.
3811f