LTC3813 Datasheet, PDF (20/32 Pages) Linear Technology – 100V Current Mode Synchronous Step-Up Controller

LTC3813
APPLICATIONS INFORMATION
Input Voltage Undervoltage Lockout
A resistor divider connected from the input supply to the
UVIN pin (see Functional Diagram) is used to program the
input supply undervoltage lockout thresholds. When the
rising voltage at UVIN reaches 0.88V, the LTC3813 turns
on, and when the falling voltage at UVIN drops below 0.8V,
the LTC3813 is shut down—providing 10% hysteresis.
The input voltage UVLO thresholds are set by the resistor
divider according to the following formulas:
V_IN,FALLING
=
0.8V
•



1+
R_UV1
R_UV2


and
V_IN,RISING
=
0.88V
•



1+
R_UV1
R_UV2


If input supply undervoltage lockout is not needed, it can
be disabled by connecting UVIN to INTV_CC.
Top MOSFET Driver Supply (C_B, D_B)
An external bootstrap capacitor C_B connected to the BOOST
pin supplies the gate drive voltage for the topside MOSFET.
This capacitor is charged through diode D_B from DRV_CC
when the switch node is low. When the top MOSFET turns
on, the switch node rises to V_OUT and the BOOST pin
rises to approximately V_OUT + DRV_CC. The boost capacitor
needs to store about 100x the gate charge required by the
top MOSFET. In most applications, 0.1μF to 0.47μF, X5R
or X7R dielectric capacitor is adequate.
The reverse breakdown of the external diode, D_B, must be
greater than V_OUT. Another important consideration for the
external diode is the reverse recovery and reverse leakage,
either of which may cause excessive reverse current to flow
at full reverse voltage. If the reverse current times reverse
voltage exceeds the maximum allowable power dissipa-
tion, the diode may be damaged. For best results, use an
ultrafast recovery diode such as the MMDL770T1.
Bottom MOSFET Driver Return Supply (BGRTN)
The bottom gate driver, BG, switches from DRV_CC to
BGRTN where BGRTN can be a voltage between ground
and –5V. Why not just keep it simple and always connect
20
BGRTN to ground? In high voltage switching converters,
the switch node dV/dt can be many volts/ns, which will
pull up on the gate of the bottom MOSFET through its
Miller capacitance. If this Miller current, times the internal
gate resistance of the MOSFET plus the driver resistance,
exceeds the threshold of the FET, shoot-through will oc-
cur. By using a negative supply on BGRTN, the BG can be
pulled below ground when turning the bottom MOSFET off.
This provides a few extra volts of margin before the gate
reaches the turn-on threshold of the MOSFET. Be aware
that the maximum voltage difference between DRV_CC and
BGRTN is 14V. If, for example, V_BGRTN = –2V, the maximum
voltage on DRV_CC pin is now 12V instead of 14V.
IC/MOSFET Driver Supplies (INTV_CC and DRV_CC)
The LTC3813 drivers are supplied from the DRV_CC pin
and the LTC3813 internal circuits from INTV_CC pin (see
Figure 1). These pins have an operating range between
6.2V and 14V. If the input voltage or another supply is not
available in this voltage range, two internal regulators are
provided to simplify the generation of this IC/driver supply
voltage as described in the next sections.
The N_DRV Pin Regulator
The N_DRV pin controls the gate of an external NMOS as
shown in Figure 9b and can be used to generate a regu-
lated 10V supply from V_IN or V_OUT. Since the NMOS is
external, it can be chosen with a BV_DSS or power rating
as high as necessary to safely derive power from a high
voltage input or output voltage. In order to generate an
INTV_CC supply that is always above the 6.2V UV threshold,
the supply connected to the drain must be greater than
6.2V + R_NDRV • 40μA + V_T.
The EXTV_CC Pin Regulator
A second low dropout regulator is available for voltages
≤ 15V. When a supply that is greater than 6.7V is con-
nected to the EXTV_CC pin, the internal LDO will regulate
10V on INTV_CC from the EXTV_CC pin voltage and will also
disable the NDRV pin regulator. This regulator is disabled
when the IC is shut down, when INTV_CC < 6.2V, or when
EXTV_CC < 6.7V.
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