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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)： | 17/25頁 |
| 文件大小： | 928K |
| 代理商： | ISL88550A |
17
FN6168.0
October 12, 2005
Undervoltage Protection (UVP)
When the output voltage drops below 70% of its regulation
voltage, and UVP is enabled (OVP/UVP = AV
DD
or REF),
the controller sets the fault latch and begins the discharge
mode (see the Shutdown and Output Discharge section).
UVP is ignored for 14ms (minimum), after startup or after a
rising edge on SHDNA#. Toggle SHDNA# or cycle AV
DD
power below 1V to clear the fault latch and restart the
controller. UVP is disabled when OVP/UVP is left open or
connected to GND (see Table 3). UVP only applies to the
Buck Output. The VTT and VTTR Outputs do not have
under voltage protection. When VDDQ is discharged to 0V
due to UVP, VTT is also discharged to 0V.
Thermal Fault Protection
The ISL88550A features a thermal fault protection circuit,
which monitors the Buck Regulator of the IC, the Linear
Regulator (VTT) and the buffered output (VTTR). When
the junction temperature of the ISL88550A rises above
+150°C, a thermal sensor activates the fault latch, pulls
POK1 low, and shuts down the buck converter using
discharge mode regardless of the OVP/UVP setting, and
VTT is also discharged to 0V. Toggle SHDNA# or cycle
AV
DD
power below 1V to reactivate the controller after the
junction temperature cools by 15°C.
Design Procedure
Firmly establish the input voltage range (V
IN
) and maximum
load current in the buck regulator before choosing a
switching frequency and inductor operating point (ripple-
current ratio or LIR). The primary design trade-off lies in
choosing a good switching frequency and inductor operating
point, and the following four factors dictate the rest of the
design:
Input Voltage Range.
The maximum value (VIN (MAX))
must accommodate the worst-case, high AC adapter
voltage. The minimum value (VIN (MIN)) must account for
the lowest battery voltage after drops due to connectors,
fuses, and battery selector switches. If there is a choice,
lower input voltages result in better efficiency.
Maximum Load Current.
There are two values to
consider. The peak load current (I
PEAK
) determines the
instantaneous component stresses and filtering
requirements and thus drives output capacitor selection,
inductor saturation rating, and the design of the current-
limit circuit. The continuous load current (I
LOAD
)
determines the thermal stresses and thus drives the
selection of input capacitors, MOSFETs, and other critical
heat-contributing components.
Switching Frequency.
This choice determines the basic
trade-off between size and efficiency. The optimal
frequency is largely a function of maximum input voltage,
due to MOSFET switching losses proportional to
frequency and VIN
2
. The optimum frequency is also a
moving target, due to rapid improvements in MOSFET
technology that are making higher frequencies more
practical.
Inductor Operating Point.
This choice provides trade-
offs: size vs. efficiency and transient response vs. output
ripple. Low inductor values provide better transient
response and smaller physical size but also result in lower
efficiency and higher output ripple due to increased ripple
currents. The minimum practical inductor value is one that
causes the circuit to operate at the edge of critical
conduction (where the inductor current just touches zero
with every cycle at maximum load). Inductor values lower
than this grant no further size-reduction benefit. The
optimum operating point is usually found between 20%
and 50% ripple current. When pulse skipping (SKIP# = low
at light loads), the inductor value also determines the load
current value at which PFM/PWM switch over occurs.
Setting the Output Voltage (Buck)
Preset Output Voltages
The ISL88550A allows the selection of common voltages
without requiring external components (Figure 25). Connect
FB to GND for a fixed 2.5V output, to AV
DD
for a fixed 1.8V
output, or connect FB directly to OUT for a fixed 0.7V output.
TABLE 3. OVP/UVP FAULT PROTECTION
OVP/UVP
DISCHARGE
UVP PROTECTION
OVP PROTECTION
AV
DD
15
internal switch ON
UGATE/LGATE is low when SHDNA# = low for normal
shutdown
Enabled.
Enabled.
UGATE pulled low and LGATE forced high if
OVP detected
OPEN
15
internal switch ON
UGATE/LGATE is low when SHDNA# = low for normal
shutdown
Disabled
Enabled.
UGATE pulled low and LGATE forced high if
OVP detected
REF
15
internal switch OFF
UGATE/LGATE is low when SHDNA# = low
Enabled.
Disabled
GND
15
internal switch OFF
UGATE/LGATE is low when SHDNA# = low
Disabled
Disabled
ISL88550A
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