LTC3786_15 Datasheet, PDF (18/34 Pages) Linear Technology – Low IQ Synchronous Boost Controller

LTC3786
Applications Information
Soft-Start (SS Pin)
The start-up of the V_OUT is controlled by the voltage on
the SS pin. When the voltage on the SS pin is less than
the internal 1.2V reference, the LTC3786 regulates the VFB
pin voltage to the voltage on the SS pin instead of 1.2V.
Soft-start is enabled by simply connecting a capacitor from
the SS pin to ground, as shown in Figure 4. An internal
10µA current source charges the capacitor, providing a
linear ramping voltage at the SS pin. The LTC3786 will
regulate the V_FB pin (and hence, V_OUT) according to the
voltage on the SS pin, allowing V_OUT to rise smoothly
from V_IN to its final regulated value. The total soft-start
time will be approximately:
t_SS
=
C_SS
•
1.2V
10µA
LTC3786
SS
C_SS
SGND
3786 F04
Figure 4. Using the SS Pin to Program Soft-Start
INTV_CC Regulator
The LTC3786 features an internal P-channel low dropout
linear regulator (LDO) that supplies power at the INTV_CC
pin from the VBIAS supply pin. INTV_CC powers the gate
drivers and much of the LTC3786’s internal circuitry. The
VBIAS LDO regulates INTV_CC to 5.4V. It can supply at least
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.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the
maximum junction temperature rating for the LTC3786
to be exceeded. The power dissipation for the IC is equal
to VBIAS • 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. For example, at 70°C ambient
18
temperature, the LTC3786 INTV_CC current is limited to
less than 20mA in the QFN package from a 40V supply:
T_J = 70°C + (20mA)(40V)(68°C/W) = 125°C
In an MSOP package, the INTV_CC current is limited to less
than 34mA from a 40V supply:
T_J = 70°C + (34mA)(40V)(40°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 VBIAS.
Topside MOSFET Driver Supply (C_B, D_B)
External bootstrap capacitors, C_B, connected to the
BOOST pin supplies the gate drive voltage for the topside
MOSFET. Capacitor C_B in the Block Diagram is charged
though external diode, D_B, from INTV_CC when the SW pin
is low. When the topside MOSFET 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_OUT and the BOOST pin follows. With the topside MOSFET
on, the boost voltage is above the output voltage: V_BOOST
= V_OUT + 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.
The topside MOSFET driver includes an internal charge
pump that delivers current to the bootstrap capacitor from
the BOOST pin. This charge current maintains the bias
voltage required to keep the top MOSFET on continuously
during dropout/overvoltage conditions. The Schottky/
silicon diode selected for the topside driver should have a
reverse leakage less than the available output current the
charge pump can supply. Curves displaying the available
charge pump current under different operating conditions
can be found in the Typical Performance Characteristics
section.
3786fa