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参数资料
| 型号： | LTC3872HDDB#TRPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 650 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | 3 X 2 MM, LEAD FREE, PLASTIC, MO-229WECD-1, DFN-8 |
| 文件页数： | 2/22页 |
| 文件大小： | 245K |
| 代理商： | LTC3872HDDB#TRPBF |
LTC3872
10
3872fb
to 30V or less, and the switch node can ring during the
turn-off of the MOSFET due to layout parasitics. Check
the switching waveforms of the MOSFET directly across
the drain and source terminals using the actual PC board
layout(notjustonalabbreadboard!)forexcessiveringing.
During the switch on-time, the control circuit limits the
maximumvoltagedropacrossthepowerMOSFETtoabout
270mV, 100mV and 170mV at low duty cycle with IPRG
tied to VIN, GND, or left floating respectively. The peak
inductorcurrentisthereforelimitedto(270mV,170mVand
100mV)/RDS(ON) depending on the status of the IPRG pin.
The relationship between the maximum load current, duty
cycle and the RDS(ON) of the power MOSFET is:
RDS(ON) ≤ VSENSE(MAX)
1– DMAX
1
+ χ
2
IO(MAX) ρT
VSENSE(MAX) is the maximum voltage drop across the
powerMOSFET.VSENSE(MAX)istypically270mV,170mVand
100mV. It is reduced with increasing duty cycle as shown
in Figure 3. The
rT term accounts for the temperature co-
efficient of the RDS(ON) of the MOSFET, which is typically
0.4%/°C. Figure 4 illustrates the variation of normalized
RDS(ON) over temperature for a typical power MOSFET.
Another method of choosing which power MOSFET to
use is to check what the maximum output current is for a
given RDS(ON),sinceMOSFETon-resistancesareavailable
in discrete values.
IO(MAX) = VSENSE(MAX)
1– DMAX
1
+ χ
2
RDS(ON) ρT
It is worth noting that the 1 – DMAX relationship between
IO(MAX) and RDS(ON) can cause boost converters with a
wide input range to experience a dramatic range of maxi-
mum input and output current. This should be taken into
consideration in applications where it is important to limit
the maximum current drawn from the input supply.
Voltage on the NGATE pin should be within –0.3V to
(VIN+0.3V) limits. Voltage stress below –0.3V and above
VIN + 0.3V can damage internal MOSFET driver, see Func-
tional Diagram. This is especially important in case of
driving MOSFETs with relatively high package inductance
(DPAK and bigger) or inadequate layout. A small Schottky
diode between NGATE pin and ground can prevent nega-
tive voltage spikes. Two small Schottky diodes can inhibit
positive and negative voltage spikes (Figure 5).
JUNCTION TEMPERATURE (°C)
–50
ρ
TNORMALIZED
ON
RESISTANCE
1.0
1.5
150
3872 F04
0.5
0
50
100
2.0
Figure 4. Normalized RDS(ON) vs Temperature
Figure 5
Figure 3. Maximum SENSE Threshold Voltage vs Duty Cycle
DUTY CYCLE (%)
1
0
MAXIMUM
CURRENT
SENSE
VOLTAGE
(mV)
50
100
150
200
250
300
20
40
60
80
3872 G03
100
IPRG = HIGH
IPRG = FLOAT
IPRG = LOW
applicaTions inForMaTion
SW
GND
VIN
NGATE
3872 F04
LTC3872
SW
GND
VIN
NGATE
LTC3872
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