LTC3788_15 Datasheet, PDF (20/32 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788
Applications Information
INTV_CC Regulators
The LTC3788 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power at
the INTV_CC pin from either the VBIAS supply pin or the
EXTV_CC pin depending on the connection of the EXTV_CC
pin. INTV_CC powers the gate drivers and much of the
LTC3788’s internal circuitry. The VBIAS LDO and the
EXTV_CC LDO regulate INTV_CC to 5.4V. Each of these can
supply a peak current of 50mA and must be bypassed to
ground with a minimum of 4.7µF ceramic capacitor. Good
bypassing is needed to supply the high transient currents
required by the MOSFET gate drivers and to prevent in-
teraction between the channels.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3788 to be
exceeded. The INTV_CC current, which is dominated by the
gate charge current, may be supplied by either the VBIAS
LDO or the EXTV_CC LDO. When the voltage on the EXTV_CC
pin is less than 4.8V, the VBIAS LDO is enabled. In this
case, power dissipation for the IC 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 3 of
the Electrical Characteristics. For example, the LTC3788
INTV_CC current is limited to less than 40mA from a 40V
supply when not using the EXTV_CC supply:
T_J = 70°C + (40mA)(40V)(34°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (PLLIN/MODE
= INTV_CC) at maximum V_IN.
When the voltage applied to EXTV_CC rises above 4.8V, the
V_IN LDO is turned off and the EXTV_CC LDO is enabled. The
EXTV_CC LDO remains on as long as the voltage applied to
EXTV_CC remains above 4.55V. The EXTV_CC LDO attempts
to regulate the INTV_CC voltage to 5.4V, so while EXTV_CC
is less than 5.4V, the LDO is in dropout and the INTV_CC
voltage is approximately equal to EXTV_CC. When EXTV_CC
is greater than 5.4V, up to an absolute maximum of 6V,
INTV_CC is regulated to 5.4V.
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Significant thermal gains can be realized by powering
INTV_CC from an external supply. Tying the EXTV_CC pin
to a 5V supply reduces the junction temperature in the
previous example from 125°C to 77°C:
T_J = 70°C + (40mA)(5V)(34°C/W) = 77°C
If more current is required through the EXTV_CC LDO than
is specified, an external Schottky diode can be added
between the EXTV_CC and INTV_CC pins. Make sure that in
all cases EXTV_CC ≤ VBIAS.
The following list summarizes possible connections for
EXTV_CC:
EXTV_CC Left Open (or Grounded). This will cause
INTV_CC to be powered from the internal 5.4V regulator
resulting in an efficiency penalty at high input voltages.
EXTV_CC Connected to an External Supply. If an external
supply is available in the 5.4V to 6V range, it may be
used to power EXTV_CC providing it is compatible with the
MOSFET gate drive requirements. Ensure that EXTV_CC
< VBIAS.
Topside MOSFET Driver Supply (C_B, D_B)
External bootstrap capacitors C_B connected to the BOOST
pins supply the gate drive voltages for the topside MOS-
FETs. Capacitor C_B in the Block Diagram is charged though
external diode D_B from INTV_CC when the SW pin is low.
When one of the topside MOSFETs is to be turned on, the
driver places the C_B voltage across the gate-source of the
desired MOSFET. This enhances the MOSFET and turns on
the topside switch. The switch node voltage, SW, rises to
V_IN and the BOOST pin follows. With the topside MOSFET
on, the boost voltage is above the input supply: V_BOOST =
V_IN + V_INTVCC. The value of the boost capacitor C_B needs
to be 100 times that of the total input capacitance of the
topside MOSFET(s). The reverse breakdown of the external
Schottky diode must be greater than V_IN(MAX).
The external diode D_B can be a Schottky diode or silicon
diode, but in either case it should have low leakage and fast
recovery. Pay close attention to the reverse leakage at high
temperatures where it generally increases substantially.
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