参数资料
| 型号： | ISL78225ANEZ-T |
| 厂商： | Intersil |
| 文件页数： | 15/22页 |
| 文件大小： | 0K |
| 描述： | IC PWM CONTROLLER |
| 标准包装： | 1,000 |
| 系列： | * |
ISL78225
V
Vfb
SOFT-START WAVEFORM (CASE B, LIGHT LOAD)
V
Vfb
SOFT-START WAVEFORM (CASE C)
Vref
NOTE: t4, t5 PERIOD ARE FROM FIGURE 5
Vref
NOTE: t4, t5 PERIOD ARE FROM FIGURE 5
0
t4
t5
t6
t7
0
t4
t5
t6
V
DIODE EMULATION
SYNCHRONOUS
OPERATION WITH
V
CYCLE-BY-CYCLE
5V
LOWER FET TURN ON
DIODE EMULATION
(PWM_INV = 0)
5V
LOWER FET TURN ON
2.5V
0
SYNCHRONOUS
PWM
0
(PWM_INV = 0)
PWM
DIODE EMULATION
OPERATION WITH
CYCLE-BY-CYCLE
DIODE EMULATION
5V
(PWM_INV = 1)
5V
V
5X10
SR SS = ------------------------ ? ------ ? (EQ. 3)
PWM
2.5V
0
5V
DRIVE_EN
0
FIGURE 16. SOFT-START WAVEFORM (CASE B, LIGHT LOAD)
Case C (V PWM_TRI = 0)
For applications that the driver cannot identify a tri-state PWM signal,
the V PWM_TRI should be connected to GND (Logic LOW), such that
the PWM signal will only be 2 levels between 0V and 5V. Then the
DRIVE_EN pin can be connected to the EN pin of the external drivers.
DRIVE_EN will be asserted when the PWM first toggles such that the
pre-bias start-up capability can be achieved. Detailed soft-start for
Case C is shown in Figure 17.
Time t 4 , t 5 : Same as the t 4 , t 5 in Figure 14, soft-start has not
Time t 6 : At the beginning of t 6 , the PWM signal will start to
switch between 0V and 5V. The driver will turn on the lower and
upper MOSFETs accordingly, and the duty cycle for lower MOSFET
will increase gradually from 0 to steady state. DRIVE_EN will be
pulled high when the first PWM toggles at the beginning of t 6 to
enable the external drivers.
(PWM_INV = 1)
PWM
0
5V
DRIVE_EN
0
FIGURE 17. SOFT-START WAVEFORM (CASE C, LIGHT LOAD)
Soft-Start Ramp Slew Rate
Calculation
The soft-start ramp slew rate SR SS is determined by the capacitor
value C SS from SS pin to GND. C SS can be calculated based on
Equation 3:
– 12
C SS
Figure 18 shows the relationship between C SS and SR SS .
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
1
10
100
C SS (nF)
FIGURE 18. SOFT-START CAPACITOR vs SLEW RATE
15
FN7909.3
February 11, 2014
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