LTC3776EUF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC3776EUF
Linear Technology
'LTC3776EUF' PDF : 28 Pages View PDF
LTC3776
APPLICATIO S I FOR ATIO
size for most LTC3776 applications, since it conducts a
relatively small average current. Larger diodes result in
additional transition losses due to their larger junction
capacitance. This diode may be omitted if the efficiency
loss can be tolerated.
CIN and COUT Selection
The selection of CIN is simplified by the 2-phase architec-
ture and its impact on the worst-case RMS current drawn
through the input network (battery/fuse/capacitor). It can
be shown that the worst-case capacitor RMS current
occurs when only one controller is operating. The control-
ler with the highest (VOUT)(IOUT) product needs to be used
in the formula below to determine the maximum RMS
capacitor current requirement. Increasing the output cur-
rent drawn from the other controller will actually decrease
the input RMS ripple current from its maximum value. The
out-of-phase technique typically reduces the input
capacitor’s RMS ripple current by a factor of 30% to 70%
when compared to a single phase power supply solution.
In continuous mode, the source current of the P-channel
MOSFET is a square wave of duty cycle (VOUT)/(VIN). To
prevent large voltage transients, a low ESR capacitor sized
for the maximum RMS current of one channel must be
used. The maximum RMS capacitor current is given by:
[ ] ( )( ) CIN
Required
IRMS
≈
IMAX
VIN
VOUT
VIN – VOUT 1/2
This formula has a maximum at VIN = 2VOUT, where IRMS
= IOUT/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that capacitor manufacturers’
ripple current ratings are often based on only 2000 hours
of life. This makes it advisable to further derate the
capacitor, or to choose a capacitor rated at a higher
temperature than required. Several capacitors may be
paralleled to meet size or height requirements in the
design. Due to the high operating frequency of the LTC3776,
ceramic capacitors can also be used for CIN. Always
consult the manufacturer if there is any question.
The benefit of the LTC3776 2-phase operation can be cal-
culated by using the equation above for the higher power
controller and then calculating the loss that would have
resulted if both controller channels switched on at the
same time. The total RMS power lost is lower when both
controllers are operating due to the reduced overlap of
current pulses required through the input capacitor’s ESR.
This is why the input capacitor’s requirement calculated
above for the worst-case controller is adequate for the
dual controller design. Also, the input protection fuse re-
sistance, battery resistance, and PC board trace resistance
losses are also reduced due to the reduced peak currents
in a 2-phase system. The overall benefit of a multiphase
design will only be fully realized when the source imped-
ance of the power supply/battery is included in the effi-
ciency testing. The sources of the P-channel MOSFETs
should be placed within 1cm of each other and share a
common CIN(s). Separating the sources and CIN may pro-
duce undesirable voltage and current resonances at VIN.
A small (0.1µF to 1µF) bypass capacitor between the chip
VIN pin and ground, placed close to the LTC3776, is also
suggested. A 10Ω resistor placed between CIN (C1) and
the VIN pin provides further isolation between the two
channels.
The selection of COUT is driven by the effective series
resistance (ESR). Typically, once the ESR requirement is
satisfied, the capacitance is adequate for filtering. The
output ripple (∆VOUT) is approximated by:
∆VOUT
≈
IRIPPLE⎛⎝⎜ ESR
+
1
8fC OUT
⎞
⎠⎟
where f is the operating frequency, COUT is the output
capacitance and IRIPPLE is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since IRIPPLE increases with input voltage.
Setting Output Voltage
The LTC3776’s channel 1 output voltage is set by an
external feedback resistor divider carefully placed across
the output, as shown in Figure 6. The regulated output
voltage is determined by:
VOUT1
=
0.6V
•
⎛⎝⎜1+
RB
RA
⎞
⎠⎟
3776f
17
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