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參數(shù)資料
| 型號： | ISL88550A |
| 廠商： | Intersil Corporation |
| 英文描述： | Synchronous Step Down Controller with Sourcing and Sinking LDO Regulator |
| 中文描述： | 同步降壓控制器，具有采購和下沉LDO穩(wěn)壓器 |
| 文件頁數(shù)： | 13/25頁 |
| 文件大?。?/td> | 928K |
| 代理商： | ISL88550A |
13
FN6168.0
October 12, 2005
in the output and ground, tend to raise the switching
frequency as the load increases. The dead-time effect
increases the effective on-time, reducing the switching
frequency as one or both dead times are added to the
effective on-time. The dead time occurs only in PWM mode,
(SKIP# = V
DD
), and during dynamic output voltage
transitions when the inductor current reverses at light or
negative load currents. With reversed inductor current, the
inductor’s EMF causes PHASE to go high earlier than
normal, extending the on-time by a period equal to the
UGATE-rising dead time. For loads above the critical
conduction point, where the dead-time effect is no longer a
factor, the actual switching frequency is:
where V
DROP1
is the sum of the parasitic voltage drops in
the inductor discharge path, including the synchronous
rectifier, the inductor, and any PC board resistances;
V
DROP2
is the sum of the resistances in the charging path,
including the high-side switch (Q1 in Typical Application
Circuit), the inductor, and any PC board resistances, and
t
ON
is the one-shot on-time (see section on On-time One-
shot (T
ON
).
Automatic Pulse-Skipping Mode (SKIP# = GND)
In skip mode, (SKIP# = GND), an inherent automatic
switchover to PFM takes place at light loads (Figure 22).
This switchover is affected by a comparator that truncates
the low-side switch on-time at the inductor current’s zero
crossing. The zero-crossing comparator differentially senses
the inductor current across the synchronous rectifier
MOSFET (Q2 in Typical Application Circuit). Once V
PGND
-
PHASE
drops below 5% of the current-limit threshold, (3mV
for the default 50mV current-limit threshold), the comparator
forces LGATE low (see Block Diagram Figure 21). This
mechanism causes the threshold between pulse-skipping
PFM and nonskipping PWM operation to coincide with the
boundary between continuous and discontinuous inductor-
current operation (also known as the critical conduction
point). The load-current level at which PFM/PWM crossover
occurs, I
LOAD(SKIP)
, is equal to one-half the peak-to-peak
ripple current, which is a function of the inductor value (see
Figure 22). This threshold is relatively constant, with only a
minor dependence on the input voltage (V
IN
).
where K is the on-time scale factor (see Table 1). For
example, in the Typical Applications Circuit (K = 1.7μs,
V
OUT
= 2.5V, V
IN
= 12V, and L = 1μH), the pulse-skipping
switchover occurs at:
The crossover point occurs at an even lower value if a
swinging (soft-saturation) inductor is used. The switching
waveforms can appear noisy and asynchronous when light
loading causes pulse-skipping operation, but this is a normal
operating condition that results in high light-load efficiency.
Trade-offs in PFM noise vs. light-load efficiency are made by
selection of inductor value. Generally, low inductor values
produce a broader efficiency vs. load curve, while higher
values result in higher full-load efficiency (assuming that the
coil resistance remains fixed), and less output voltage ripple.
Penalties for using higher inductor values include larger
physical size and degraded load-transient response,
especially at low input voltage levels.
DC output accuracy specifications refer to the threshold of
the error comparator. When the inductor is in continuous
conduction, the ISL88550A regulates the valley of the output
ripple, so the actual DC output voltage is higher than the trip
level by 50% of the output ripple voltage. In discontinuous
conduction (SKIP# = GND and I
LOAD
< I
LOAD(SKIP)
), the
output voltage has a DC regulation level higher than the
error comparator threshold by approximately 1.5% due to
slope compensation.
FIGURE 22. PULSE SKIPPING/DISCONTINUOUS
CROSSOVER POINT
)
2
DROP
V
IN
ON
1
DROP
V
OUT
V
SW
f
t
V
+
+
=
(
)
V
×
=
IN
OUT
V
IN
OUT
2
SKIP
LOAD
I
V
L
K
V
A
68
.
V
12
V
5
V
12
μH
1
2
μs
7
×
V
5
=
×
ON-TIME
0
TIME
I
PEAK
L
V
IN
- V
OUT
t
=
I
LOAD
=I
PEAK
/2
I
I
ISL88550A
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