LTC3720 Datasheet, PDF (15/24 Pages) Linear Technology – Single Phase VRM8.5 Current Mode Step-Down Controller

APPLICATIO S I FOR ATIO
Table 1. VID Output Voltage Programming
VID4
VID3
VID2
VID1
VID0
0
0
0
0
0
0
0
0
0
1
0
0
0
1
0
0
0
0
1
1
0
0
1
0
0
0
0
1
0
1
0
0
1
1
0
0
0
1
1
1
0
1
0
0
0
0
1
0
0
1
0
1
0
1
0
0
1
0
1
1
0
1
1
0
0
0
1
1
0
1
0
1
1
1
0
0
1
1
1
1
1
0
0
0
0
1
0
0
0
1
1
0
0
1
0
1
0
0
1
1
1
0
1
0
0
1
0
1
0
1
1
0
1
1
0
1
0
1
1
1
1
1
0
0
0
1
1
0
0
1
1
1
0
1
0
1
1
0
1
1
1
1
1
0
0
1
1
1
0
1
1
1
1
1
0
1
1
1
1
1
V_OUT (V)
1.250V
1.275V
1.200V
1.225V
1.150V
1.175V
1.100V
1.125V
1.050V
1.075V
1.800V
1.825V
1.750V
1.775V
1.700V
1.725V
1.650V
1.675V
1.600V
1.625V
1.550V
1.575V
1.500V
1.525V
1.450V
1.475V
1.400V
1.425V
1.350V
1.375V
1.300V
1.325V
LTC3720
operation may cause the LTC3720 to exceed its maximum
junction temperature rating or RMS current rating. Most
of the supply current drives the MOSFET gates unless an
external EXTV_CC source is used. In continuous mode
operation, this current is I_GATECHG = f(Q_g(TOP) + Q_g(BOT)).
The junction temperature can be estimated from the
equations given in Note 3 of the Electrical Characteristics.
For example, the LTC3720EGN is limited to less than
19mA from a 30V supply:
T_J = 70°C + (19mA)(30V)(95°C/W) = 125°C
For larger currents, consider using an external supply with
the EXTV_CC pin.
EXTV_CC Connection
The EXTV_CC pin can be used to provide MOSFET gate drive
and control power from an external source during normal
operation. Whenever the EXTV_CC pin is above 4.7V the
internal 5V regulator is shut off and an internal 50mA P-
channel switch connects the EXTV_CC pin to INTV_CC. INTV_CC
power is supplied from EXTV_CC until this pin drops below
4.5V. Do not apply more than 7V to the EXTV_CC pin and
ensure that EXTV_CC ≤ V_IN. The following list summarizes
the possible connections for EXTV_CC:
1. EXTV_CC grounded. INTV_CC is always powered from the
internal 5V regulator.
2. EXTV_CC connected to an external supply. A high effi-
ciency supply compatible with the MOSFET gate drive
requirements (typically 5V) can improve overall
efficiency.
3. EXTV_CC connected to an output derived boost network.
The low voltage output can be boosted using a charge
pump or flyback winding to greater than 4.7V. The system
will start-up using the internal linear regulator until the
boosted output supply is available.
3720f
15