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ISL78225 Datasheet, PDF (15/21 Pages) Intersil Corporation – 4-Phase Interleaved Boost PWM Controller with Light Load Efficiency Enhancement
ISL78225
SOFT-START WAVEFORM (CASE B, LIGHT LOAD)
V
Vfb
Vref
0 t4 t5
V
5V
2.5V
NOTE: t4, t5 PERIOD ARE FROM FIGURE 5
t6
DIODE EMULATION
LOWER FET TURN ON
t7
SYNCHRONOUS
OPERATION WITH
CYCLE-BY-CYCLE
DIODE EMULATION
(PWM_INV = 0)
PWM
0
SYNCHRONOUS
DIODE EMULATION
OPERATION WITH
CYCLE-BY-CYCLE
DIODE EMULATION
5V
(PWM_INV = 1)
PWM
2.5V
0
SOFT-START WAVEFORM (CASE C)
V
Vfb
Vref
0 t4 t5
NOTE: t4, t5 PERIOD ARE FROM FIGURE 5
t6
V
LOWER FET TURN ON
5V
(PWM_INV = 0)
PWM
0
5V
(PWM_INV = 1)
PWM
0
5V
DRIVE_EN
5V
DRIVE_EN
0
FIGURE 16. SOFT-START WAVEFORM (CASE B, LIGHT LOAD)
Case C (VPWM_TRI = 0)
For applications that the driver cannot identify a tri-state PWM signal,
the VPWM_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 t4, t5: Same as the t4, t5 in Figure 14, soft-start has not
started yet; see “Operation Initialization Before Soft-Start” on
page 13 for detailed description.
Time t6: At the beginning of t6, 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 t6 to
enable the external drivers.
0
FIGURE 17. SOFT-START WAVEFORM (CASE C, LIGHT LOAD)
Soft-Start Ramp Slew Rate
Calculation
The soft-start ramp slew rate SRSS is determined by the capacitor
value CSS from SS pin to GND. CSS can be calculated based on
Equation 3:
SRSS
=
5----X----1----0----–--1----2-
CSS
⎛
⎝
μ--V--s--⎠⎞
(EQ. 3)
Figure 18 shows the relationship between CSS and SRSS.
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
1
10
100
CSS (nF)
FIGURE 18. SOFT-START CAPACITOR vs SLEW RATE
15
FN7909.0
December 15, 2011