LTC3350_15 Datasheet, PDF (29/46 Pages) Linear Technology – High Current Supercapacitor Backup Controller and System Monitor

LTC3350
Applications Information
body diodes of the MOSFET switches from turning on,
storing charge during the non-overlap time and requiring
a reverse recovery period that could cost as much as 3%
in efficiency at high V_IN. One or both diodes can be omit-
ted if the efficiency loss can be tolerated. The diode can
be rated for about one-third to one-fifth of the full load
current since it is on for only a fraction of the duty cycle.
Larger diodes result in additional switching losses due to
their larger junction capacitance. In order for the diodes
to be effective, the inductance between them and the top
and bottom MOSFETs must be as small as possible. This
mandates that these components be placed next to each
other on the same layer of the PC board.
Top MOSFET Driver Supply (C_B, D_B)
An external bootstrap capacitor, C_B, connected to the BST
pin supplies the gate drive voltage for the top MOSFET.
Capacitor C_B, in Figure 8, is charged though an external
diode, D_B, from DRV_CC when the SW pin is low. The value
of the bootstrap capacitor, C_B, needs to be 20 times that
of the total input capacitance of the top MOSFET.
With the top MOSFET on, the BST voltage is above the
system supply rail:
V_BST = V_OUT + V_DRVCC
The reverse break down of the external diode, D_B, must
be greater than V_OUT(MAX) + V_DRVCC(MAX).
The step-up converter can briefly run nonsynchronously
when used in conjunction with the output ideal diode. Dur-
ing this time the BST to SW voltage can pump up to voltages
exceeding 5.5V if D_B is a Schottky diode. Fast switching PN
diodes are recommended due to their low leakage and junc-
tion capacitance. A Schottky diode can be used if the step-up
converter runs synchronous throughout backup mode.
INTV_CC/DRV_CC and IC Power Dissipation
The LTC3350 features a low dropout linear regulator
(LDO) that supplies power to INTV_CC from the V_OUT sup-
ply. INTV_CC powers the gate drivers (when connected to
DRV_CC) and much of the LTC3350’s internal circuitry. The
LDO regulates the voltage at the INTV_CC pin to 5V. The
LDO can supply a maximum current of 50mA and must
be bypassed to ground with a minimum of 1μF when not
connected to DRV_CC. DRV_CC should have at least a 2.2μF
ceramic or low ESR electrolytic capacitor. No matter what
type of bulk capacitor is used on DRV_CC, an additional
0.1μF ceramic capacitor placed directly adjacent to the
DRV_CC pin is highly recommended. 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 maxi-
mum junction temperature rating for the LTC3350 to be
exceeded. The INTV_CC current, which is dominated by the
gate charge current, is supplied by the 5V LDO.
Power dissipation for the IC in this case is highest and is
approximately equal to (V_OUT) • (I_Q + I_G), where I_Q is the
non-switching quiescent current of ~4mA and I_G is gate
charge current. The junction temperature can be estimated
by using the equations given in Note 2 of the Electrical
Characteristics. For example, the I_G supplied by the INTV_CC
LDO is limited to less than 42mA from a 35V supply in the
QFN package at a 70°C ambient temperature:
T_J = 70°C + (35V)(4mA + 42mA)(34°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the INTV_CC LDO current must be checked while
operating in continuous conduction mode at maximum
V_OUT.
BST
LTC3350
SW
DRV_CC
INTV_CC
C_B
D_B
0.1µF
1µF
OPT
>2.2µF
3350 F07
Figure 8. Bootstrap Capacitor/Diode and DRV_CC Connections
The power dissipation in the IC is drastically reduced if
DRV_CC is powered from an external LDO. In this case the
power dissipation in the IC is equal to power dissipation
due to I_Q and the power dissipated in the gate drivers,
(V_DRVCC) • (I_G). Assuming the external DRV_CC LDO output
is 5V and is supplying 42mA to the gate drivers, the junc-
tion temperature rises to only 82°C:
T_J = 70°C + [(35V)(4mA)+(5V)(42mA)](34°C/W) = 82°C
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