LTC3703_15 Datasheet, PDF (16/34 Pages) Linear Technology – 100V Synchronous Switching Regulator Controller

LTC3703
Applications Information
Since ∆I_L increases with input voltage, the output ripple
is highest at maximum input voltage. ESR also has a sig-
nificant effect on the load transient response. Fast load
transitions at the output will appear as voltage across the
ESR of C_OUT until the feedback loop in the LTC3703 can
change the inductor current to match the new load current
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 through-
hole capacitors. The OS-CON (organic semiconductor
dielectric) capacitor available from Sanyo has the lowest
product of ESR and size of any aluminum electrolytic at
a somewhat higher price. An additional ceramic capacitor
in parallel with OS-CON capacitors is recommended 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, special
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 output voltage is set by a resistor divider
according to the following formula:
V_OUT
=
0.8V


1+
R1_
R2 ^
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.
MOSFET Driver Supplies (DRV_CC and BOOST)
The LTC3703 drivers are supplied from the DRV_CC and
BOOST pins (see Figure 3), 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 9V 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 10. If the output voltage is between 10V
and 15V, the output can be used to directly power the
drivers as shown in Figure 10a. If the output is below
10V, Figure 10b shows an easy way to boost the supply
voltage to a sufficient level. This boost circuit uses the
LT1613 in a ThinSOT™ package and a chip inductor for
minimal extra area (<0.2in²). Two other possible schemes
are an extra winding on the inductor (Figure 10c) or a
capacitive charge pump (Figure 10d). All the circuits
shown in Figure 10 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 exist-
ing 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 10.
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
topside 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
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