LTC3866 Datasheet, PDF (22/36 Pages) Linear Technology – Current Mode Synchronous Controller for Sub Milliohm DCR Sensing

LTC3866
Applications Information
V_OUT1
V_OUT1
V_OUT2
V_OUT2
TIME
(8a) Coincident Tracking
TIME
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(8b) Ratiometric Tracking
Figure 8. Two Different Modes of Output Voltage Tracking
V_OUT1
TO
TK/SS2
PIN
R3 R1
TO
TO
V_FB1 V_FB2
PIN
PIN
R4 R2
V_OUT2
R3
R4
V_OUT1
TO
TK/SS2
PIN
R1
TO
TO
V_FB1 V_FB2
PIN
PIN
R2
V_OUT2
R3
R4
(9a) Coincident Tracking Setup
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(9b) Ratiometric Tracking Setup
Figure 9. Setup and Coincident and Ratiometric Tracking
some trade-offs exist. The ratiometric mode saves a pair
of resistors, but the coincident mode offers better output
regulation. Under ratiometric tracking, when the master
controller’s output experiences dynamic excursion (under
load transient, for example), the slave controller output
will be affected as well. For better output regulation, use
the coincident tracking mode instead of ratiometric.
INTV_CC (LDO) and EXTV_CC
The LTC3866 features a true PMOS LDO that supplies
power to INTV_CC from the V_IN supply. INTV_CC powers the
gate drivers and much of the LTC3866’s internal circuitry.
The LDO regulates the voltage at the INTV_CC pin to 5.5V
when V_IN is greater than 6V. EXTV_CC connects to INTV_CC
through a P-channel MOSFET and can supply the needed
power when its voltage is higher than 4.7V. Either of these
can supply a peak current of 100mA and must be bypassed
to ground with a minimum of 4.7µF ceramic capacitor or
low ESR electrolytic capacitor. No matter what type of bulk
capacitor is used, an additional 0.1µF ceramic capacitor
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placed directly adjacent to the INTV_CC and PGND pins is
highly recommended. Good bypassing is needed to sup-
ply the high transient currents required by the MOSFET
gate drivers. High input voltage applications in which
large MOSFETs are being driven at high frequencies may
cause the maximum junction temperature rating for the
LTC3866 to be exceeded. The INTV_CC current, which is
dominated by the gate charge current, may be supplied by
either the 5.5V LDO or EXTV_CC. When the voltage on the
EXTV_CC pin is less than 4.5V, the LDO is enabled. Power
dissipation for the IC in this case is highest and is equal
to V_IN • I_INTVCC. 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 tables. For example, the LTC3866
INTV_CC current is limited to less than 39mA from a 38V
supply in the UF package and not using the EXTV_CC supply
with a 70°C ambient temperature:
T_J = 70°C + (39mA)(38V)(37°C/W) ≅ 125°C
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