LTC3788-1 Datasheet, PDF (19/28 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788-1
APPLICATIONS INFORMATION
INTV_CC Regulators
The LTC3788-1 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
LTC3788-1’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 LTC3788-1 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 LTC3788-1
INTV_CC current is limited to less than 15mA from a 40V
supply when not using the EXTV_CC supply:
T_J = 70°C + (15mA)(40V)(90°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 4.7V, 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.
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 regu-
lator resulting in an efficiency penalty at high input
voltages.
EXTV_CC Connected to an External Supply. If an exter-
nal 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).
Fault Conditions: Overtemperature Protection
At higher temperatures, or in cases where the internal
power dissipation causes excessive self-heating on chip
(such as an INTV_CC short to ground), the overtemperature
shutdown circuitry will shut down the LTC3788-1. When
the junction temperature exceeds approximately 170°C,
the overtemperature circuitry disables the INTV_CC LDO,
causing the INTV_CC supply to collapse and effectively shut
down the entire LTC3788-1 chip. Once the junction tem-
perature drops back to approximately 155°C, the INTV_CC
LDO turns back on. Long term overstress (T_J > 125°C)
should be avoided as it can degrade the performance or
shorten the life of the part.
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