LTC3703IGN-5 Linear Technology, LTC3703IGN-5 Datasheet - Page 16

• Manufacturer Part Number: LTC3703IGN-5
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,CMOS,SSOP,16PIN,PLASTIC
• Manufacturer: Linear Technology

LTC3703
APPLICATIO S I FOR ATIO
Since ∆I
highest at maximum input voltage. ESR also has a signifi-
cant effect on the load transient response. Fast load
transitions at the output will appear as voltage across the
ESR of C
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
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 output voltage is set by a resistor divider
according to the following formula:
The external resistor divider is connected to the output as
shown in the Functional Diagram, allowing remote voltage
16
V
OUT
L
OUT
=
increases with input voltage, the output ripple is
0 8 1
.
until the feedback loop in the LTC3703 can
V
⎛
⎜
⎝
+
U
R
R
2
1
⎞
⎟
⎠
U
W
U
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
The LTC3703 drivers are supplied from the DRV
BOOST pins (see Figure 3), which have an absolute
maximum voltage of 15V. If the main supply voltage, V
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
area (<0.2 in
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
supply must be capable of supplying the required gate
driver current which can be estimated from:
This equation for I
the circuit components shown in Figure 10.
An external bootstrap capacitor, C
BOOST pin supplies the gate drive voltage for the topside
MOSFETs. Capacitor C
diode, D
the top side MOSFET is turned on, the driver places the C
voltage across the gate-source of the top MOSFET. The
switch node voltage, SW, rises to V
follows. With the topside MOSFET on, the boost voltage
I
DRVCC
B
, from the DRV
= (f)(Q
TM
package and a chip inductor for minimal extra
2
). Two other possible schemes are an extra
G(TOP)
DRVCC
IN
. When using an existing supply, the
+ Q
CC
B
is also useful for properly sizing
is charged through external
G(BOTTOM)
supply when SW is low. When
CC
and BOOST)
IN
B
)
, connected to the
and the BOOST pin
CC
3703fa
and
IN
B
,