LTC1530IS8-1.9 查看數據表（PDF） - Linear Technology
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LTC1530IS8-1.9 Datasheet PDF : 24 Pages
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LTC1530
APPLICATIO S I FOR ATIO
OPTIONAL FOR
VIN > 6.5V
13V
+
1N5243B
10µF
MBR0530T1 MBR0530T1 VIN
PVCC
0.22µF
+
CIN
G1
G2
LTC1530
Q1
LO
+
Q2
VOUT
CO
1530 F07
Figure 7. Doubling Charge Pump
Power MOSFETs
Two N-channel power MOSFETs are required for synchro-
nous LTC1530 circuits. They should be selected based
primarily on threshold voltage and on-resistance consid-
erations. Thermal dissipation is often a secondary con-
cern in high efficiency designs. The required MOSFET
threshold should be determined based on the available
power supply voltages and/or the complexity of the gate
drive charge pump scheme. In 5V input designs where a
12V supply is used to power PVCC, standard MOSFETs
with RDS(ON) specified at VGS = 5V or 6V can be used with
good results. The current drawn from the 12V supply
varies with the MOSFETs used and the LTC1530’s operat-
ing frequency, but is generally less than 50mA.
LTC1530 applications that use a 5V VIN voltage and a
doubling charge pump to generate PVCC do not provide
enough gate drive voltage to fully enhance standard
power MOSFETs. Under this condition, the effective
MOSFET RDS(ON) may be quite high, raising the dissipa-
tion in the FETs and reducing efficiency. In addition,
power supply start-up problems can occur with standard
power MOSFETs. These start-up problems can occur for
two reasons. First, if the MOSFET is not fully enhanced,
the higher effective RDS(ON) causes the LTC1530 to acti-
vate current limit at a much lower level than the desired
trip point. Second, standard MOSFETs have higher GATE
threshold voltages than logic level MOSFETs, thereby
increasing the PVCC voltage required to turn them on. A
MOSFET whose RDS(ON) is rated at VGS = 4.5V does not
necessarily have a logic level MOSFET GATE threshold
voltage. Logic level FETs are the recommended choice for
5V-only systems. Logic level FETs can be fully enhanced
with a doubler charge pump and will operate at maximum
efficiency. Note that doubler charge pump designs run-
ning from supplies higher than 6.5V should include a
Zener diode clamp at PVCC to prevent transients from
exceeding the absolute maximum rating of the pin.
After the MOSFET threshold voltage is selected, choose
the RDS(ON) based on the input voltage, the output voltage,
allowable power dissipation and maximum output cur-
rent. In a typical LTC1530 buck converter circuit, operat-
ing in continuous mode, the average inductor current is
equal to the output load current. This current flows through
either Q1 or Q2 with the power dissipation split up accord-
ing to the duty cycle:
DC(Q1) = VOUT
VIN
( ) DC(Q2) = 1− VOUT = VIN − VOUT
VIN
VIN
The RDS(ON) required for a given conduction loss can now
be calculated by rearranging the relation P = I2R.
[ ] RDS(ON)Q1 =
PMAX(Q1)
DC(Q1)
IMAX2
( )[ ] VIN PMAX(Q1)
=
( ) VOUT
IMAX2
[ ] RDS(ON)Q2 =
PMAX(Q2)
DC
(Q2)
IMAX2
( )[ ] VIN PMAX(Q2)
=
( ) VIN
−
VOUT
IMAX2
11
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