LTC3728EUH#TR Linear Technology, LTC3728EUH#TR Datasheet - Page 25

• Manufacturer Part Number: LTC3728EUH#TR
• Description: IC SW REG SYNC STP-DN DUAL 32QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3728EUH#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 5.5 V
• Current - Output: 3A
• Frequency - Switching: 250kHz ~ 550kHz
• Voltage - Input: 3.5 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Other names: LTC3728EUHTR

APPLICATIONS INFORMATION
Voltage Positioning
Voltage positioning can be used to minimize peak-to-peak
output voltage excursions under worst-case transient
loading conditions. The open-loop DC gain of the control
loop is reduced depending upon the maximum load step
specifi cations. Voltage positioning can easily be added to
the LTC3728 by loading the I
having a Thevenin equivalent voltage source equal to the
midpoint operating voltage range of the error amplifi er, or
1.2V (see Figure 8).
The resistive load reduces the DC loop gain while main-
taining the linear control range of the error amplifi er. The
maximum output voltage deviation can theoretically be
reduced to half, or alternatively, the amount of output
capacitance can be reduced for a particular application.
A complete explanation is included in Design Solutions
10 (see www.linear.com).
Effi ciency Considerations
The percent effi ciency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the effi ciency and which change would
produce the most improvement. Percent effi ciency can
be expressed as:
where L1, L2, etc. are the individual losses as a percent-
age of input power.
%Effi ciency = 100% – (L1 + L2 + L3 + ...)
Figure 8. Active Voltage Positioning
Applied to the LTC3728
INTV
CC
R
R
T2
T1
R
C
C
C
TH
I
TH
pin with a resistive divider
LTC3728
3728 F08
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most
of the losses in LTC3728 circuits: 1) LTC3728 V
rent (including loading on the 3.3V internal regulator),
2) INTV
MOSFET transition losses.
1. The V
2. INTV
3. I
DC supply current given in the Electrical Characteristics
table, which excludes MOSFET driver and control cur-
rents; the second is the current drawn from the 3.3V
linear regulator output. V
a small (<0.1%) loss.
control currents. The MOSFET driver current results
from switching the gate capacitance of the power
MOSFETs. Each time a MOSFET gate is switched from
low to high to low again, a packet of charge dQ
moves from INTV
a current out of INTV
than the control circuit current. In continuous mode,
I
charges of the topside and bottom side MOSFETs.
Supplying INTV
input from an output-derived source will scale the V
current required for the driver and control circuits by
a factor of (Duty Cycle)/(Effi ciency). For example, in a
20V to 5V application, 10mA of INTV
in approximately 2.5mA of V
the mid-current loss from 10% or more (if the driver
was powered directly from V
fuse (if used), MOSFET, inductor, current sense resis-
tor, and input and output capacitor ESR. In continuous
mode, the average output current fl ows through L and
R
and the synchronous MOSFET. If the two MOSFETs
have approximately the same R
tance of one MOSFET can simply be summed with the
resistances of L, R
For example, if each R
GATECHG
2
SENSE
R losses are predicted from the DC resistances of the
CC
CC
IN
, but is “chopped” between the topside MOSFET
current is the sum of the MOSFET driver and
regulator current, 3) I
current has two components: the fi rst is the
= f(Q
T
CC
CC
Q
SENSE
power through the EXTV
B
), where Q
to ground. The resulting dQ/dt is
CC
DS(ON)
and ESR to obtain I
that is typically much larger
IN
current typically results in
IN
IN
) to only a few percent.
= 30mΩ, R
2
T
DS(ON)
current. This reduces
R losses, 4) Topside
and Q
CC
LTC3728
, then the resis-
current results
B
are the gate
L
2
CC
= 50mΩ,
R losses.
25
IN
switch
3728fg
cur-
IN