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參數(shù)資料
| 型號(hào)： | 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)： | 14/25頁 |
| 文件大小： | 928K |
| 代理商： | ISL88550A |
14
FN6168.0
October 12, 2005
Force PWM Mode (SKIP# = AV
DD
)
The low-noise forced-PWM mode, (SKIP# = AV
DD
), disables
the zero-crossing comparator, which controls the low-side
switch on-time. This forces the low-side gate drive waveform
to constantly be the complement of the high-side gate-drive
waveform, so the inductor current reverses at light loads
while UGATE maintains a duty factor of V
OUT
/V
IN
. Forced-
PWM mode keeps the switching frequency fairly constant.
However, forced-PWM operation comes at a cost where the
no-load V
DD
bias current remains between 2mA and 20mA
due to the external MOSFETs gate charge and switching
frequency. Forced-PWM mode is most useful for reducing
audio frequency noise, improving load-transient response,
and providing sink current capability for dynamic output
voltage adjustment.
Current Limit Buck Regulator (ILIM)
VALLEY CURRENT LIMIT
The current-limit circuit for the Buck Regulator portion of the
ISL88550A employs a unique “valley” current sensing
algorithm that senses the voltage drop across PHASE and
PGND1 and uses the on-resistance of the rectifying
MOSFET (Q2 in the Typical Application Circuit) as the
current sensing element. If the magnitude of the current
sense signal is above the valley current-limit threshold, the
PWM controller is not allowed to initiate a new cycle (Figure
24). With Valley Current Limit sensing, the actual peak
current is greater than the valley current-limit threshold by an
amount equal to the inductor current ripple. Therefore, the
exact current limit characteristic and maximum load
capability are a function of the current-sense resistance,
inductor value, and input voltage. When combined with the
undervoltage protection circuit, this current-limit method is
effective in almost every circumstance
.
In forced-PWM mode, the ISL88550A also implements a
negative current limit to prevent excessive reverse inductor
currents when the Buck Regulator output is sinking current.
The negative current-limit threshold is set to approximately
120% of the positive current limit and tracks the positive
current limit when V
ILIM
is adjusted. The current-limit
threshold is adjusted with an external resistor-divider at ILIM.
A 2μA to 20μA divider current is recommended for accuracy
and noise immunity.
The current-limit threshold adjustment range is from 25mV to
200mV. In the adjustable mode, the current limit threshold
voltage (from PHASE to PGND1) is precisely 1/10th the
voltage seen at ILIM. The threshold defaults to 50mV when
ILIM is connected to AV
DD
. The logic threshold for
switchover to the 50mV default value is approximately AV
DD
- 1V.
Carefully observe the PC board layout guidelines to ensure
that noise and DC errors do not corrupt the differential
current-sense signals seen between PHASE and PGND1.
TABLE 1. APPROXIMATE K-FACTOR ERRORS
T
ON
SETTING
TYPICAL K
FACTOR (μs)
K-FACTOR
ERROR
(10%)
MINIMUM V
IN
AT V
OUT
= 2.5V
(h = 1.5, SEE DROPOUT
PERFORMANCE SECTION)
TYPICAL
APPLICATION
COMMENTS
200kHz
(T
ON
= AV
DD
)
5.0
±10
3.15
4-Cell Li+
Notebook
Use for absolute best efficiency
300kHz
(T
ON
= OPEN)
3.3
±10
3.47
4-Cell Li+
Notebook
Considered mainstream by current
standards
450kHz
(T
ON
= REF)
2.2
±12.5
4.13
3-Cell Li+
Notebook
Useful in 3 cell systems for lighter loads
600kHz
(T
ON
= GND)
1.7
±12.5
5.61
+5V input
Good operating point for compound
buck designs or desktop circuts.
ISL88550A
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