LTC1709EG-8 Linear Technology, LTC1709EG-8 Datasheet - Page 14

• Manufacturer Part Number: LTC1709EG-8
• Description: IC REG SW 2PH SYNC STPDWN 36SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1709EG-8

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 1.3 ~ 3.5 V
• Current - Output: 3A
• Voltage - Input: 4 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 36-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Frequency - Switching: -

APPLICATIO S I FOR ATIO
LTC1709-8/LTC1709-9
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during a
short-circuit when the synchronous switch is on close to
100% of the period.
The term (1 + ) is generally given for a MOSFET in the
form of a normalized R
voltage MOSFETs. C
MOSFET characteristics. The constant k = 1.7 can be
used to estimate the contributions of the two terms in the
main switch dissipation equation.
The Schottky diodes, D1 and D2 shown in Figure 1
conduct during the dead-time between the conduction of
the two large power MOSFETs. This helps prevent the
body diode of the bottom MOSFET from turning on,
storing charge during the dead-time, and requiring a
reverse recovery period which would reduce efficiency. A
1A to 3A Schottky (depending on output current) diode is
generally a good compromise for both regions of opera-
tion due to the relatively small average current. Larger
diodes result in additional transition losses due to their
larger junction capacitance.
C
In continuous mode, the source current of each top
N-channel MOSFET is a square wave of duty cycle V
V
RMS current must be used. The details of a closed form
equation can be found in Application Note 77. Figure 4
shows the input capacitor ripple current for a 2-phase
configuration with the output voltage fixed and input
voltage varied. The input ripple current is normalized
against the DC output current. The graph can be used in
place of tedious calculations. The minimum input ripple
current can be achieved when the input voltage is twice the
output voltage
In the graph of Figure 4, the 2-phase local maximum input
RMS capacitor currents are reached when:
where k = 1, 2
14
IN
IN
= 0.005/ C can be used as an approximation for low
. A low ESR input capacitor sized for the maximum
V
and C
V
OUT
IN
OUT
2
k
Selection
4
1
U
RSS
DS(ON)
U
is usually specified in the
vs temperature curve, but
W
U
OUT
/
These worst-case conditions are commonly used for
design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s 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 also be paralleled to
meet size or height requirements in the design. Always
consult the capacitor manufacturer if there is any
question.
It is important to note that the efficiency loss is propor-
tional to the input RMS current squared and therefore a
2-phase implementation results in 75% less power loss
when compared to a single phase design. Battery/input
protection fuse resistance (if used), PC board trace and
connector resistance losses are also reduced by the
reduction of the input ripple current in a 2-phase system.
The required amount of input capacitance is further
reduced by the factor, 2, due to the effective increase in
the frequency of the current pulses.
The selection of C
series resistance (ESR). Typically once the ESR require-
ment has been met, the RMS current rating generally far
exceeds the I
output ripple ( V
V
OUT
Figure 4. Normalized RMS Input Ripple Current
vs Duty Factor for 1 and 2 Output Stages
0.6
0.5
0.4
0.3
0.2
0.1
0
0.1
RIPPLE(P-P)
I
RIPPLE
0.2
OUT
OUT
0.3
DUTY FACTOR (V
) is determined by:
is driven by the required effective
ESR
0.4
requirements. The steady state
1-PHASE
2-PHASE
0.5
16
OUT
0.6
fC
/V
1
IN
OUT
0.7
)
0.8
170989 F04
0.9