参数资料
| 型号： | ISL85001IRZ-T |
| 厂商： | Intersil |
| 文件页数： | 13/17页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 1A 12DFN |
| 标准包装： | 6,000 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 0.6 V ~ 19 V |
| 输入电压： | 4.5 V ~ 25 V |
| PWM 型： | 电压模式 |
| 频率 - 开关： | 500kHz |
| 电流 - 输出： | 1A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 12-VFDFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | 12-DFN（4x3） |
ISL85001
Use only specialized low-ESR capacitors intended for switching-
regulator applications for the bulk capacitors. The bulk capacitor’s
with a 20% derating factor. The power dissipation is shown in
Equation 5:
P D [ W ] = I OUT ? V D ? ? 1 – ---------------- ?
ESR will determine the output ripple voltage and the initial voltage
drop after a high slew-rate transient. An aluminum electrolytic
capacitor’s ESR value is related to the case size with lower ESR
? V OUT ?
? V IN ?
(EQ. 5)
V IN - V OUT V OUT
Δ V OUT = Δ I x ESR
Δ I = (EQ. 3)
V IN
-------------- × ? I OUT
+ ------ × ? ----------------------------- × -------------- ? ?
I RMS
?
V IN ? ?
?
L × f s
V IN
available in larger case sizes. However, the Equivalent Series
Inductance (ESL) of these capacitors increases with case size and
can reduce the usefulness of the capacitor to high slew-rate
transient loading. Unfortunately, ESL is not a specified parameter.
Work with your capacitor supplier and measure the capacitor’s
impedance with frequency to select a suitable component. In most
cases, multiple electrolytic capacitors of small case size perform
better than a single large case capacitor.
Output Inductor Selection
The output inductor is selected to meet the output voltage ripple
requirements and minimize the converter’s response time to the
load transient. The inductor value determines the converter’s
ripple current and the ripple voltage is a function of the ripple
current. The ripple voltage and current are approximated by
Equation 3:
x
Fs x L
Increasing the value of inductance reduces the ripple current and
voltage. However, the large inductance values reduce the
converter’s response time to a load transient.
One of the parameters limiting the converter’s response to a
load transient is the time required to change the inductor
current. Given a sufficiently fast control loop design, the
ISL85001 will provide either 0% or 80% 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. Equation 4 gives the approximate
response time interval for application and removal of a transient
load:
L x I TRAN L x I TRAN
t RISE = t FALL = (EQ. 4)
V IN - V OUT V OUT
where: I TRAN is the transient load current step, t RISE is the
response time to the application of load, and t FALL is the
response time to the removal of load. The worst case response
time can be either at the application or removal of load. Be sure
to check Equation 4 at the minimum and maximum output levels
for the worst case response time.
Rectifier Selection
Current circulates from ground to the junction of the MOSFET and
the inductor when the high-side switch is off. As a consequence,
the polarity of the switching node is negative with respect to
ground. This voltage is approximately -0.5V (a Schottky diode drop)
during the off-time. The rectifier's rated reverse breakdown voltage
must be at least equal to the maximum input voltage, preferably
13
where V D is the voltage of the Schottky diode = 0.5V to 0.7V
Input Capacitor Selection
Use a mix of input bypass capacitors to control the voltage
overshoot across the MOSFETs. Use small ceramic capacitors for
high frequency decoupling and bulk capacitors to supply the
current needed each time the switching MOSFET turns on. Place
the small ceramic capacitors physically close to the MOSFET VIN
pins (switching MOSFET drain) and the Schottky diode anode.
The important parameters for the bulk input capacitance are the
voltage rating and the RMS current rating. For reliable operation,
select bulk capacitors with voltage and current ratings above the
maximum input voltage and largest RMS current required by the
circuit. Their voltage rating should be at least 1.25x greater than
the maximum input voltage, while a voltage rating of 1.5x is a
conservative guideline. For most cases, the RMS current rating
requirement for the input capacitor of a buck regulator is
approximately 1/2 the DC load current.
The maximum RMS current required by the regulator may be
closely approximated through Equation 6:
V OUT 2 1 V IN – V OUT V OUT 2
=
MAX MAX 12
(EQ. 6)
For a through-hole design, several electrolytic capacitors may be
needed. For surface mount designs, solid tantalum capacitors
can be used, but caution must be exercised with regard to the
capacitor surge current rating. These capacitors must be capable
of handling the surge-current at power-up. Some capacitor series
available from reputable manufacturers are surge current tested.
Feedback Compensation
Figure 21 highlights the voltage-mode control loop for a
synchronous-rectified buck converter. The output voltage (V OUT )
is regulated to the Reference voltage level. The error amplifier
output (V E/A ) is compared with the oscillator (OSC) triangular
wave to provide a pulse-width modulated (PWM) wave with an
amplitude of V IN at the PHASE node. The PWM wave is smoothed
by the output filter (L O and C O ).
The modulator transfer function is the small-signal transfer
function of V OUT /V E/A . This function is dominated by a DC Gain
and the output filter (L O and C O ), with a double pole break
frequency at F LC and a zero at F ESR . The DC Gain of the
modulator is simply the input voltage (V IN ) divided by the
peak-to-peak oscillator voltage Δ V OSC .
FN6769.2
May 29, 2012
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