LTC3811_15 Datasheet, PDF (16/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
OPERATION (Refer to the Functional Diagram)
be increased to 10μF. Because of the high peak current
capability of the gate driver, it is essential that this capacitor
be placed as close as possible to DRV_CC and PGND pins,
and on the same PCB layer as the IC.
The INTV_CC pin supplies power to all of the low voltage
analog circuitry and is electrically isolated from DRV_CC. The
INTV_CC supply is normally derived from DRV_CC through
an RC filter, in order to prevent gate driver supply noise
from coupling into sensitive analog control circuitry. Typi-
cal values for this RC filter consist of a 1Ω resistor from
DRV_CC to INTV_CC and a 0.1μF low ESR ceramic capacitor
from INTV_CC to SGND. The INTV_CC capacitor should be
placed as close as possible to the INTV_CC and SGND pins
and on the same PCB layer as the IC.
A third power supply pin, EXTV_CC, serves as an auxiliary
input for applications where the power dissipation in the
internal LDO is excessive, or where maximum efficiency
is essential. This configuration is shown in Figure 2. When
the EXTV_CC pin is left open or is connected to a voltage
less than 4.5V, the internal 6V LDO supplies DRV_CC power
from V_IN. If EXTV_CC is tied to an external power supply
greater than 4.5V, however, the 6V LDO is turned off and
power is supplied to DRV_CC through a 5Ω PMOS switch
from EXTV_CC. For 4.5V < EXTV_CC < 7V this PMOS switch
is on and DRV_CC is approximately equal to EXTV_CC. Using
the EXTV_CC pin allows the gate driver and control power to
be derived from a high efficiency external source, dramati-
cally reducing power dissipation on the IC.
V_IN
V_IN
EXTV_CC
4.5V +
AUX 5V
SUPPLY
–
6V
LDO
DRV_CC
INTV_CC
SGND
BIAS
GATE DRIVER SUPPLY
ANALOG SUPPLY
–
V_FB
SS/TRACK ₊EA
0.600V
3811 F02
Using an External 5V Supply to Measure Dynamic
Quiescent Current
When a voltage above 4.5V is applied to the EXTV_CC pin, the
internal LDO in the LTC3811 is switched off and the power
is supplied by the external 5V power supply as shown in
Figure 2. Under these conditions, the quiescent current
at the V_IN pin of the IC is very low (less than 1mA), and
most of the current required to power the analog control
circuitry and the gate drivers flows into the EXTV_CC pin. As
a result, this auxiliary supply can be used as a diagnostic
tool in order to measure the total current for thermal
calculations. In order to match the actual condition when
the internal LDO is on, the voltage applied to EXTV_CC when
the measurements are taken should be 6V (the same as
the regulated LDO output voltage).
Once the total quiescent current for the application is
known, the power dissipation, P_D, on the IC will be ap-
proximately I_EXTVCC times V_IN, since the gate drive current
and control circuitry quiescent current would be required
to flow through the V_IN pin. The junction temperature of
the IC can then be estimated using the following well-
known formula:
T_J = T_A + (P_D • R_θJA)
If the maximum junction temperature is close to the Abso-
lute Maximum Rating for the particular device being used,
the use of an auxiliary supply and the EXTV_CC pin may be
required. Alternatively, lower gate charge MOSFETs should
be used or the switching frequency should be reduced.
Operation at Low Supply Voltage
The LTC3811 control circuit has a minimum input volt-
age of 4.5V, making it a good choice for applications that
experience low supply conditions. However, care should
be taken to determine the minimum gate drive supply
voltage in order to choose the optimum power MOSFETs.
Important parameters that can affect the minimum gate
drive voltage are the minimum input voltage (V_IN), the
LDO dropout voltage, and the EXTV_CC supply voltage, if
an external gate drive supply is being used.
Figure 2. Supplying Power to the LTC3811 from EXTV_CC
3811f
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