参数资料
| 型号： | ISL78225ANEZ-T |
| 厂商： | Intersil |
| 文件页数： | 14/22页 |
| 文件大小： | 0K |
| 描述： | IC PWM CONTROLLER |
| 标准包装： | 1,000 |
| 系列： | * |
ISL78225
Soft-Start Process for Different
Modes (Refer to Table 1)
At the beginning of soft-start, the SS pin voltage will start
ramping up from a voltage equal to the FB voltage. The soft-start
V
Vfb
SOFT-START WAVEFORM (CASE A)
DIODE EMULATION
period ends when the SS pin voltage reaches the lower
power-good threshold that is 80% of the lower value of VREF2
or 2V.
Case A (V MODE = VCC, V PWM_TRI = VCC)
Figure 15 shows the pre-bias start-up PWM waveform for Case A
in Table 1. The V PWM_TRI = VCC so the PWM can output a tri-level
signal, which the external drivers need to identify, and
0
V
5V
2.5V
Vref
t4
t5
NOTE: t4, t5 PERIOD ARE FROM FIGURE 5
t6 t7
SYNCHRONOUS
OPERATION
LOWER FET TURN ON
(PWM_INV = 0)
PWM
V MODE = VCC to ban the automatic phase dropping function.
0
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 SS pin has already been
pre-biased to a value very close to the V FB , so that the internal
reference signal will start from the voltage close to the FB pin.
5V
2.5V
0
DIODE EMULATION
SYNCHRONOUS
OPERATION
(PWM_INV = 1)
PWM
This scheme will eliminate the internal delay for a non pre-biased
application.
5V
The DRIVE_EN pin, which is connected to the enable pins of the
external drivers, will be pulled high when first PWM toggles at the
beginning of t 6; as a result external drivers will start working. The
PWM signals will switch between tri-level and low. The driver will
only turn on the lower MOSFET accordingly, and the duty cycle
will increase gradually from 0 to steady state. The synchronous
MOSFET (Upper FET for Boost converter) will never turn on during
this time, so diode emulation can be achieved during the start-up
and in turn prevent negative current flowing from output to input.
Time t 7 : Soft-start finishes at the beginning of t 7 . The PWMs will
change to a 2-level 0V to 5V switching signal and the
synchronous MOSFET will be turned on.
14
DRIVE_EN
0
FIGURE 15. SOFT-START WAVEFORM (CASE A)
Case B (V MODE < 4V, V PWM_TRI = VCC, Light
Load Condition)
The only difference between Case A and Case B start-up
waveforms is that at light load, Case B can drop phases and have
cycle-by-cycle diode emulation at PWM1 and PWM2.
For Case B applications, where good light load efficiency is
always preferred, the ISL78225 provides three light load
efficiency enhancement methods. When the load current
reduces, the ISL78225 will first assert the automatic phase
dropping function to reduce the active phase number according
to the load level. The minimum active phase number is two. If the
load current further reduces even when running at two-phase
operation, the ISL78225 will assert a second method by utilizing
cycle-by-cycle diode emulation. During this time the IC will sense
the inductor current, and when the current is approximately zero,
it will turn off the synchronous MOSFET. If the load current is
further reduced to deep light load operation, pulse skipping
function will kick in to optimize the overall efficiency.
FN7909.3
February 11, 2014
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