LTC3703-5_15 Datasheet, PDF (16/32 Pages) Linear Technology – 60V Synchronous Switching Regulator Controller

LTC3703-5
APPLICATIO S I FOR ATIO
value. Typically, once the ESR requirement is satisfied the
capacitance is adequate for filtering and has the required
RMS current rating.
Manufacturers such as Nichicon, Nippon Chemi-Con and
Sanyo should be considered for high performance
throughhole capacitors. The OS-CON (organic semicon-
ductor dielectric) capacitor available from Sanyo has the
lowest product of ESR and size of any aluminum electro-
lytic at a somewhat higher price. An additional ceramic
capacitor in parallel with OS-CON capacitors is recom-
mended to reduce the effect of their lead inductance.
In surface mount applications, multiple capacitors placed
in parallel may be required to meet the ESR, RMS current
handling and load step requirements. Dry tantalum, spe-
cial polymer and aluminum electrolytic capacitors are
available in surface mount packages. Special polymer
capacitors offer very low ESR but have lower capacitance
density than other types. Tantalum capacitors have the
highest capacitance density but it is important to only use
types that have been surge tested for use in switching
power supplies. Several excellent surge-tested choices
are the AVX TPS and TPSV or the KEMET T510 series.
Aluminum electrolytic capacitors have significantly higher
ESR, but can be used in cost-driven applications providing
that consideration is given to ripple current ratings and
long term reliability. Other capacitor types include
Panasonic SP and Sanyo POSCAPs.
Output Voltage
The LTC3703-5 output voltage is set by a resistor divider
according to the following formula:
V_OUT = 0.8V^{⎛⎝⎜}1+ _RR21^{⎞⎠⎟}
The external resistor divider is connected to the output as
shown in the Functional Diagram, allowing remote voltage
sensing. The resultant feedback signal is compared with
the internal precision 800mV voltage reference by the
error amplifier. The internal reference has a guaranteed
tolerance of ±1%. Tolerance of the feedback resistors will
add additional error to the output voltage. 0.1% to 1%
resistors are recommended.
16
MOSFET Driver Supplies (DRV_CC and BOOST)
The LTC3703-5 drivers are supplied from the DRV_CC and
BOOST pins (see Figure 2), which have an absolute
maximum voltage of 15V. If the main supply voltage, V_IN,
is higher than 15V a separate supply with a voltage
between 5V and 15V must be used to power the drivers. If
a separate supply is not available, one can easily be
generated from the main supply using one of the circuits
shown in Figure 9. If the output voltage is between 5V and
15V, the output can be used to directly power the drivers
as shown in Figure 9a. If the output is below 5V, Figure 9b
shows an easy way to boost the supply voltage to a
sufficient level. This boost circuit uses the LT1613 in a
ThinSOT^TM package and a chip inductor for minimal extra
area (<0.2 in²). Two other possible schemes are an extra
winding on the inductor (Figure 9c) or a capacitive charge
pump (Figure 9d). All the circuits shown in Figure 9
require a start-up circuit (Q1, D1 and R1) to provide driver
power at initial start-up or following a short-circuit. The
resistor R1 must be sized so that it supplies sufficient base
current and zener bias current at the lowest expected value
of V_IN. When using an existing supply, the supply must be
capable of supplying the required gate driver current
which can be estimated from:
I_DRVCC = (f)(Q_G(TOP) + Q_G(BOTTOM))
This equation for I_DRVCC is also useful for properly sizing
the circuit components shown in Figure 9.
An external bootstrap capacitor, C_B, connected to the
BOOST pin supplies the gate drive voltage for the topside
MOSFETs. Capacitor C_B is charged through external
diode, D_B, from the DRV_CC supply when SW is low. When
the top side MOSFET is turned on, the driver places the C_B
voltage across the gate-source of the top MOSFET. 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_DRVCC. The
value of the boost capacitor C_B needs to be 100 times that
of the total input capacitance of the top side MOSFET(s).
The reverse breakdown of the external diode, D_B, must be
greater than V_IN(MAX). Another important consideration
for the external diode is the reverse recovery and reverse
leakage, either of which may cause excessive reverse
ThinSOT is a tradmark of Linear Technology Corporation.
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