LTC3786EUD#PBF Linear Technology, LTC3786EUD#PBF Datasheet - Page 20

• Manufacturer Part Number: LTC3786EUD#PBF
• Description: IC, DC-DC CONV, 550kHz, QFN16
• Manufacturer: Linear Technology

Specifications of LTC3786EUD#PBF

• Primary Input Voltage: 38V
• No. Of Outputs: 1
• Output Voltage: 60V
• No. Of Pins: 16
• Operating Temperature Range: -40°C To +125°C
• Peak Reflow Compatible (260 C): Yes
• Switching Frequency Max: 550kHz
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
LTC3786
In forced continuous mode, if the duty cycle falls below
what can be accommodated by the minimum on-time,
the controller will begin to skip cycles but the output will
continue to be regulated. More cycles will be skipped when
V
the top MOSFET continuously on. The minimum on-time
for the LTC3786 is approximately 110ns.
Efficiency Considerations
The percent efficiency 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 efficiency and which change would
produce the greatest improvement. Percent efficiency
can be expressed as:
where L1, L2, etc., are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, five main sources usually account for most of the
losses in LTC3786 circuits: 1) IC VBIAS current, 2) INTV
regulator current, 3) I
tion losses and 5) Body diode conduction losses.
1. The VBIAS current is the DC supply current given in the
2. INTV
3. DC I
20
IN
Electrical Characteristics table, which excludes MOSFET
driver and control currents. VBIAS current typically
results in 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
of INTV
circuit current. In continuous mode, I
Q
and bottom side MOSFETs.
MOSFETs, sensing resistor, inductor and PC board
traces and cause the efficiency to drop at high output
currents.
%Efficiency = 100% – (L1 + L2 + L3 + ...)
increases. Once V
B
), where Q
2
CC
CC
R losses. These arise from the resistances of the
CC
to ground. The resulting dQ/dt is a current out
current is the sum of the MOSFET driver and
that is typically much larger than the control
T
and Q
2
IN
R losses, 4) Bottom MOSFET transi-
B
are the gate charges of the topside
rises above V
OUT
GATECHG
, the loop keeps
= f(Q
T
CC
+
4. Transition losses apply only to the bottom MOSFET(s),
5. Body diode conduction losses are more significant at
Other hidden losses, such as copper trace and internal
battery resistances, can account for an additional efficiency
degradation in portable systems. It is very important
to include these system-level losses during the design
phase.
Checking Transient Response
The regulator loop response can be checked by looking at
the load current transient response. Switching regulators
take several cycles to respond to a step in load current.
When a load step occurs, V
to ΔI
of C
generating the feedback error signal that forces the regula-
tor to adapt to the current change and return V
steady-state value. During this recovery time V
be monitored for excessive overshoot or ringing, which
would indicate a stability problem. OPTI-LOOP
sation allows the transient response to be optimized over
a wide range of output capacitance and ESR values. The
availability of the ITH pin not only allows optimization of
control loop behavior, but it also provides a DC-coupled
and AC-filtered closed-loop response test point. The DC
step, rise time and settling at this test point truly reflects the
closed-loop response. Assuming a predominantly second
order system, phase margin and/or damping factor can be
estimated using the percentage of overshoot seen at this
pin. The bandwidth can also be estimated by examining the
rise time at the pin. The ITH external components shown
in the Figure 8 circuit will provide an adequate starting
point for most applications.
and become significant only when operating at low input
voltages. Transition losses can be estimated from:
higher switching frequency. During the dead time, the loss
in the top MOSFETs is I
At higher switching frequency, the dead time becomes
a good percentage of switching cycle and causes the
efficiency to drop.
OUT
LOAD(ESR)
Transition Loss = 1.7
. ΔI
LOAD
, where ESR is the effective series resistance
also begins to charge or discharge C
( )
L
• V
OUT
DS
V
, where V
shifts by an amount equal
OUT
V
IN
3
I
MAX
DS
• C
is around 0.7V.
RSS
®
OUT
compen-
OUT
• f
to its
OUT
can
3786f