LTC3814-5_15 Datasheet, PDF (17/30 Pages) Linear Technology – 60V Current Mode Synchronous Step-Up Controller

LTC3814-5
APPLICATIONS INFORMATION
Input Capacitor Selection
The input capacitor of a boost converter is less critical
than the output capacitor, due to the fact that the inductor
is in series with the input and the input current waveform
is continuous (see Figure 8b). The input voltage source
impedance determines the size of the input capacitor,
which is typically in the range of 10μF to 100μF. A low
ESR capacitor is recommended though not as critical as
for the output capacitor.
The RMS input capacitor ripple current for a boost con-
verter is:
I_RMS(CIN)
= 0.3 •
V_IN(MIN)
L•f
• D_MAX
Please note that the input capacitor can see a very high
surge current when a battery is suddenly connected to
the input of the converter and solid tantalum capacitors
can fail catastrophically under these conditions. Be sure
to specify surge-tested capacitors!
Output Voltage
The LTC3814-5 output voltage is set by a resistor divider
according to the following formula:
V_OUT
=
0.8V



1+
R_FB1
R_FB2


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 less than ±1%. Tolerance of the feedback
resistors will add additional error to the output voltage.
0.1% to 1% resistors are recommended.
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 INTV_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 + INTV_CC. The boost capacitor needs
to store about 100 times 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 al-
lowable power dissipation, the diode may be damaged.
For best results, use an ultrafast recovery diode such as
the MMDL770T1.
IC/MOSFET Driver Supplies (INTV_CC)
The LTC3814-5 drivers and the LTC3814-5 internal circuits
are supplied from the INTV_CC pin (see Figure 1). These
pins have an operating range between 4.2V 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 5.5V 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 4.2V UV threshold,
the supply connected to the drain must be greater than
4.2V + 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 4.7V is con-
nected to the EXTV_CC pin, the internal LDO will regulate
5.5V 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 < 4.2V, or when
EXTV_CC < 4.7V.
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