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| 型号： | ISL6232EVAL1 |
| 厂商： | Intersil |
| 文件页数： | 19/25页 |
| 文件大小： | 0K |
| 描述： | EVALUATION BOARD 1 ISL6232 |
| 标准包装： | 1 |
| 主要目的： | DC/DC，LDO 步降 |
| 输出及类型： | 4，非隔离 |
| 输入电压： | 5.5 ~ 25 V |
| 稳压器拓扑结构： | 降压 |
| 频率 - 开关： | 300kHz |
| 板类型： | 完全填充 |
| 已供物品： | 板 |
| 已用 IC / 零件： | ISL6232 |
ISL6232
TABLE 3. SUMMARY FOR VARIOUS OPERATION MODES
Application Information
MODE
Shutdown
Standby
Soft-Start
CONDITION
SHDN# = Low.
SHDN# = High.
EN3 = EN5 = Low.
LDO5>UVLO EN3
or/and EN5 enabled.
COMMENT
All circuitry off.
LDO5, LDO3, and 2V
reference active. LGATE
stays high.
Output voltage ramps up
in 1.2ms.
This section describes how to select the external
components including the inductor, input and output
capacitors, switching MOSFETs, current sensing resistors
and loop compensator design.
The inductor selection has to accommodate trade-offs
between cost, size and efficiency. For example, the lower the
inductance, the smaller the inductor size, but ripple current is
Normal
Operation
Discharge
SHDN# = High. EN3 and All circuitry is running.
EN5 enabled.
Either output is still high Discharging the output
in standby mode. through an internal 20 Ω
switch from PHASE to
PGND. One output may
still operate while the
other is in discharge
mode. LDO5 active.
higher; this results in higher AC losses in the magnetic core
and the windings, which decrease the system efficiency. On
the other hand, the higher inductance results in lower ripple
current and smaller output filter capacitors, but higher DCR
(DC resistance of the inductor) loss and slower transient
response. Practical inductor design is based on the inductor
ripple current being ±(15 to 20)% of the maximum operating
DC current at maximum input voltage. The required
inductance can be calculated from Equation 5:
Undervoltage Either output is below
Lower side MOSFET is
Δ I L
Protection
75% of nominal after a
20ms blanking time and
output enabled.
latched on after discharge
mode terminates. LDO5 is
active. Reset by toggling
V IN – VOUT VOUT
L = ---------------------------------- -----------------
VIN f s
(EQ. 5)
EN3, EN5, SHDN#, VIN
POR.
where V IN is input voltage, V OUT is the output voltage, Δ I L is
Overvoltage Either output voltage is
Protection 13% higher than the
nominal.
Low side MOSFET is
forced high and high side
MOSFET is forced low.
the inductor ripple current and f s is the switching frequency.
The practical inductor ripple current is chosen at 30% of the
output current: Δ I L = 30% ? I OUT
LINEAR REGULATORS AND 2V REFERENCE
For V IN = 12V, V OUT = 5V, I OUT = 5A, and f s = 300kHz,
L = ----------------- ------------------------------------- 3 - = 6.5 μ H
In ISL6232, there are two internal regulators available, which
are LDO5 (5V) and LDO3 (3.3V). Once LDO5 is higher than
12 – 5 5
0.3 × 5 12 × 300 × 10
(EQ. 6)
(EQ. 7)
I OC = -----------------
4.3V, it provides power for buck controllers, 2V reference,
and all the other blocks powered by VCC. The maximum
guaranteed output current that both LDO5 and LDO3
regulators can supply is 100mA. The real maximum current
drawn from the LDOs is determined by the maximum power
dissipation allowed in the package. A short-circuit or
overcurrent limit protection, 170mA (typ), is implemented for
both LDO5 and LDO3. Bypass LDO5 and LDO3 with a 4.7μF
ceramic capacitor.
When OUT5 is larger than the LDO5 switch-over threshold
(4.78V) and after soft-start is finished, LDO5 is shorted to
OUT5 through an internal 2 Ω switch and the LDO5 regulator
is disabled to reduce the power dissipation. Similarly, when
OUT3 is larger than the LDO3 switch-over threshold (3.0V)
and after soft-start is finished, LDO3 is shorted to OUT3
through an internal 2.5 Ω switch and LDO3 is turned off. All
the internal blocks (powered by VCC) get the power from the
high-efficiency switching power supply instead of the linear
regulator.
The reference voltage, REF, is 2V with a ±1.5% accuracy.
REF provides the reference voltage, 0.8V, for buck
controllers. REF is bypassed to GND with a 0.22μF
capacitor.
19
Ferrite core inductors are often the best choice since they
are optimized at 300kHz to 600kHz operation with low core
loss. The inductor must be large enough not to saturate at
the overcurrent limit I OC
95 mV
R CS
One important factor is that the smaller the inductance, the
faster the transient response. One of the parameters limiting
the converters response to load transient is the time required
to change the inductor current. Given a sufficiently fast
control loop design, the ISL6232 can provide either
approximately 5% or 95% duty cycle in response to a load
transient. The response time is the time required to slew the
inductor current from an initial current value to the transient
current level. During this interval the difference between the
inductor current and the transient current level must be
supplied by the output capacitor. Minimizing the response
time can minimize the output capacitance required. The
response time to a transient is different for the application of
load and the removal of load.
FN9116.1
April 20, 2009
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