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| 型号： | LTC1757-2CMS8 |
| 英文描述： | Industrial Control IC |
| 中文描述： | 工业控制IC |
| 文件页数： | 2/12页 |
| 文件大小： | 178K |
| 代理商： | LTC1757-2CMS8 |
10
LTC1701/LTC1701B
SW
VFB
VIN
ITH/RUN
GND
LTC1701
D1
L1
R2
R1
COUT
1701 F04
VOUT
VIN
1
2
3
5
4
CIN
CC
RC
RS
BOLD LINES INDICATE HIGH CURRENT PATHS
+
Figure 4. LTC1701 Layout Diagram (See Board Layout Checklist)
APPLICATIO S I FOR ATIO
WU
U
The following table lists thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 3/32" FR-4 board with one ounce
copper.
Table 1. Measured Thermal Resistance
COPPER AREA
THERMAL RESISTANCE
TOPSIDE*
BACKSIDE
BOARD AREA
θJA
2500mm2
125
°C/W
1000mm2
2500mm2
125
°C/W
225mm2
2500mm2
130
°C/W
100mm2
2500mm2
135
°C/W
50mm2
2500mm2
150
°C/W
*Device is mounted on topside.
Calculating Junction Temperature
In a majority of applications, the LTC1701 does not dissi-
pate much heat due to its high efficiency. However, in
applications where the switching regulator is running at
high duty cycles or the part is in dropout with the switch
turned on continuously (DC), some thermal analysis is
required. The goal of the thermal analysis is to determine
whether the power dissipated by the regulator exceeds the
maximum junction temperature. The temperature rise is
given by:
TRISE = PD θJA
where PD is the power dissipated by the regulator and θJA
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature is given by:
TJ = TRISE + TAMBIENT
As an example, consider the case when the LTC1701 is in
dropout at an input voltage of 3.3V with a load current of
0.5A. The ON resistance of the P-channel switch is approxi-
mately 0.30
. Therefore, power dissipated by the part is:
PD = I2 RDS(ON) = 75mW
The SOT package junction-to-ambient thermal resistance,
θJA, will be in the range of 125°C/W to 150°C/W. Therefore,
the junction temperature of the regulator operating in a
25
°C ambient temperature is approximately:
TJ = 0.075 150 + 25 = 36°C
Remembering that the above junction temperature is ob-
tained from a RDS(ON) at 25°C, we might recalculate the
junction temperature based on a higher RDS(ON) since it
increases with temperature. However, we can safely as-
sume that the actual junction temperature will not exceed
the absolute maximum junction temperature of 125
°C.
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1701. These items are also illustrated graphically in the
layout diagram of Figure 4. Check the following in your
layout:
1. Does the capacitor CIN connect to the power VIN (Pin 5)
and GND (Pin 2) as close as possible? This capacitor
provides the AC current to the internal P-channel MOSFET
and its driver.
2. Is the Schottky diode closely connected between the
ground (Pin 2) and switch output (Pin 1)?
3. Are the COUT, L1 and D1 closely connected? The Schottky
anode should connect directly to the input capacitor ground.
4. The resistor divider, R1 and R2, must be connected
between the (+) plate of COUT and a ground line terminated
near GND (Pin 2). The feedback signal FB should be routed
away from noisy components and traces, such as the SW
line (Pin 1).
5. Keep sensitive components away from the SW pin. The
input capacitor CIN, the compensation capacitor CC and all
the resistors R1, R2, RC and RS should be routed away from
the SW trace and the components L1 and D1.
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