LTC3867_15 Datasheet, PDF (27/36 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3867
APPLICATIONS INFORMATION
LTC3867
V_IN
INTV_CC
R_VIN
1Ω
+ _CINTVCC
4.7µF
5V
C_IN
3867 F12
Figure 12. Setup for a 5V Input
Topside MOSFET Driver Supply (C_B, D_B)
External bootstrap capacitor, C_B, connected to the BOOST
pin supplies the gate drive voltages for the topside MOSFET.
Capacitor C_B in the Functional Diagram is charged though
external diode D_B from INTV_CC when the SW pin is low.
When the topside MOSFET is to be turned on, the driver
places the C_B voltage across the gate source of the MOSFET.
This enhances the MOSFET and turns on the topside switch.
The switch node voltage, SW, rises to V_IN and the BOOST
pin follows. With the topside MOSFET on, the boost volt-
age is above the input supply:
V_BOOST = V_IN + V_INTVCC – V_DB
The value of the boost capacitor, C_B, needs to be 100 times
that of the total input capacitance of the topside MOSFET(s).
The reverse breakdown of the external Schottky diode
must be greater than V_IN(MAX). When adjusting the gate
drive level, the final arbiter is the total input current for
the regulator. If a change is made and the input current
decreases, then the efficiency has improved. If there is
no change in input current, then there is no change in
efficiency.
Setting Output Voltage
The LTC3867 output voltage is set by an external feed-
back resistive divider carefully placed across the output,
as shown in Figure 13. The regulated output voltage is
determined by:
V_OUT
=
0.6V
•


1+
R_B
R_A


To improve the frequency response, a feedforward ca-
pacitor, C_FF , may be used. Great care should be taken to
V_OUT
LTC3867
V_FB
R_B
C_FF
R_A
3867 F13
Figure 13. Setting Output Voltage
route the V_FB line away from noise sources, such as the
inductor or the SW line.
If the diffamp is used, then the resistor divider center tap
should connect to the noninverting input of the diffamp,
DIFF⁺. DIFFOUT should then be shorted to V_FB.
Fault Conditions: Current Limit and Current Foldback
The LTC3867 includes current foldback to help limit load
current when the output is shorted to ground. If the out-
put falls below 50% of its nominal output level, then the
maximum sense voltage is progressively lowered from its
maximum programmed value to one-third of the maximum
value. Foldback current limiting is disabled during the soft-
start or tracking up using the TK/SS pin. It is not disabled
for internal soft-start. Under short-circuit conditions with
very low duty cycles, the LTC3867 will begin cycle skipping
in order to limit the short-circuit current. In this situation
the bottom MOSFET will be dissipating most of the power
but less than in normal operation. The short circuit ripple
current is determined by the minimum on-time t_ON(MIN) of
the LTC3867 (≈65ns), the input voltage and inductor value:
∆I_L(SC)
=
t_ON(MIN)
•
V_IN
L
The resulting short-circuit current is:
I_SC
=


1/3
V_{SENSE(MAX )}
R_SENSE
–
1
2

∆I_{L(SC) }
After a short, or while starting with internal soft-start, make
sure that the load current takes the folded-back current
limit into account.
3867f
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