LT3757(RevB) Просмотр технического описания (PDF) - Linear Technology
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LT3757 Datasheet PDF : 36 Pages
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LT3757
Applications Information
For applications where the SHDN/UVLO pin is only used
as a logic input, the SHDN/UVLO pin can be connected
directly to the input voltage VIN for always-on operation.
INTVCC Regulator Bypassing and Operation
An internal, low dropout (LDO) voltage regulator produces
the 7.2V INTVCC supply which powers the gate driver, as
shown in Figure 1. If a low input voltage operation is ex-
pected (e.g., supplying power from a lithium-ion battery
or a 3.3V logic supply), low threshold MOSFETs should
be used. The LT3757 contains an undervoltage lockout
comparator A8 and an overvoltage lockout comparator
A9 for the INTVCC supply. The INTVCC undervoltage (UV)
threshold is 2.7V (typical), with 100mV hysteresis, to
ensure that the MOSFETs have sufficient gate drive voltage
before turning on. The logic circuitry within the LT3757 is
also powered from the internal INTVCC supply.
The INTVCC overvoltage (OV) threshold is set to be 17.5V
(typical) to protect the gate of the power MOSFET. When
INTVCC is below the UV threshold, or above the OV thresh-
old, the GATE pin will be forced to GND and the soft-start
operation will be triggered.
The INTVCC regulator must be bypassed to ground im-
mediately adjacent to the IC pins with a minimum of 4.7µF
ceramic capacitor. Good bypassing is necessary to supply
the high transient currents required by the MOSFET gate
driver.
In an actual application, most of the IC supply current is
used to drive the gate capacitance of the power MOSFET.
The on-chip power dissipation can be a significant concern
when a large power MOSFET is being driven at a high
frequency and the VIN voltage is high. It is important to
limit the power dissipation through selection of MOSFET
and/or operating frequency so the LT3757 does not exceed
its maximum junction temperature rating. The junction
temperature TJ can be estimated using the following
equations:
TJ = TA + PIC • θJA
TA = ambient temperature
θJA = junction-to-ambient thermal resistance
PIC = IC power consumption
= VIN • (IQ + IDRIVE)
IQ = VIN operation IQ = 1.6mA
IDRIVE = average gate drive current = f • QG
f = switching frequency
QG = power MOSFET total gate charge
The LT3757 uses packages with an Exposed Pad for en-
hanced thermal conduction. With proper soldering to the
Exposed Pad on the underside of the package and a full
copper plane underneath the device, thermal resistance
(θJA) will be about 43°C/W for the DD package and 40°C/W
for the MSE package. For an ambient board temperature of
TA = 70°C and maximum junction temperature of 125°C,
the maximum IDRIVE (IDRIVE(MAX)) of the DD package can
be calculated as:
IDRIVE(MAX)
=
(TJ − TA)
(θJA • VIN)
− IQ
=
1.28W
VIN
−
1.6mA
The LT3757 has an internal INTVCC IDRIVE current limit
function to protect the IC from excessive on-chip power
dissipation. The IDRIVE current limit decreases as the VIN
increases (see the INTVCC Minimum Output Current vs VIN
graph in the Typical Performance Characteristics section).
If IDRIVE reaches the current limit, INTVCC voltage will fall
and may trigger the soft-start.
Based on the preceding equation and the INTVCC Minimum
Output Current vs VIN graph, the user can calculate the
maximum MOSFET gate charge the LT3757 can drive at
a given VIN and switch frequency. A plot of the maximum
QG vs VIN at different frequencies to guarantee a minimum
4.5V INTVCC is shown in Figure 2.
As illustrated in Figure 2, a trade-off between the operating
frequency and the size of the power MOSFET may be needed
in order to maintain a reliable IC junction temperature.
Prior to lowering the operating frequency, however, be
sure to check with power MOSFET manufacturers for their
most recent low QG, low RDS(ON) devices. Power MOSFET
manufacturing technologies are continually improving, with
newer and better performance devices being introduced
almost yearly.
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