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| 型号： | NCP1580DR2 |
| 厂商： | ON Semiconductor |
| 文件页数： | 8/12页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM VM 8-SOIC |
| 产品变化通告： | Product Discontinuation 01/Oct/2008 |
| 标准包装： | 2,500 |
| PWM 型： | 电压模式 |
| 输出数： | 1 |
| 频率 - 最大： | 412kHz |
| 占空比： | 95% |
| 电源电压： | 4.5 V ~ 13.2 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 8-SOIC（0.154"，3.90mm 宽） |
| 包装： | 带卷 (TR) |
NCP1580
APPLICATION SECTION
Input Capacitor Selection
The input capacitor has to sustain the ripple current
D VOUT * DISCHARGE +
2
D IOUT2
COUT (VIN
LOUT
D * VOUT)
,
produced during the on time of the upper MOSFET, so it
must have a low ESR to minimize the losses. The RMS value
of this ripple is:
IinRMS + IOUT D (1 * D) ,
where D is the duty cycle, I inRMS is the input RMS current,
and I OUT is the load current. The equation reaches its
maximum value with D = 0.5. Losses in the input capacitors
can be calculated with the following equation:
where V OUT?DISCHARGE is the voltage deviation of V OUT
due to the effects of discharge, L OUT is the output inductor
value and V IN is the input voltage.
It should be noted that D V OUT?DISCHARGE and
D V OUT?ESR are out of phase with each other, and the larger
of these two voltages will determine the maximum deviation
of the output voltage (neglecting the effect of the ESL).
Inductor Selection
PCIN + ESRCIN
IinRMS2,
Both mechanical and electrical considerations influence
the selection of an output inductor. From a mechanical
SlewRateLOUT + IN
Iinrush +
COUT VOUT
where P CIN is the power loss in the input capacitors and
ESR CIN is the effective series resistance of the input
capacitance. Due to large dI/dt through the input capacitors,
electrolytic or ceramics should be used. If a tantalum must
be used, it must be surge protected. Otherwise, capacitor
failure could occur.
Calculating Input Startup Current
To calculate the input startup current, the following
equation can be used.
,
tSS
where I inrush is the input current during startup, C OUT is
the total output capacitance, V OUT is the desired output
voltage, and t SS is the internal soft?start interval.
If the inrush current is higher than the steady state input
current during max load, then the input fuse should be rated
accordingly, if one is used.
Output Capacitor Selection
The output capacitor is a basic component for the fast
perspective, smaller inductor values generally correspond to
smaller physical size. Since the inductor is often one of the
largest components in the regulation system, a minimum
inductor value is particularly important in
space?constrained applications. From an electrical
perspective, the maximum current slew rate through the
output inductor for a buck regulator is given by:
V * VOUT
LOUT
This equation implies that larger inductor values limit the
regulator ’s ability to slew current through the output
inductor in response to output load transients. Consequently,
output capacitors must supply the load current until the
inductor current reaches the output load current level. This
results in larger values of output capacitance to maintain
tight output voltage regulation. In contrast, smaller values of
inductance increase the regulator ’s maximum achievable
slew rate and decrease the necessary capacitance, at the
expense of higher ripple current. The peak?to?peak ripple
current is given by the following equation:
response of the power supply. In fact, during load transient,
for the first few microseconds it supplies the current to the
Ipk?pkLOUT +
VOUT(1 * D)
LOUT 350 kHZ
,
load. The controller immediately recognizes the load
transient and sets the duty cycle to maximum, but the current
slope is limited by the inductor value.
During a load step transient the output voltage initially
drops due to the current variation inside the capacitor and the
ESR. (neglecting the effect of the effective series inductance
(ESL)):
D VOUT?ESR + D IOUT ESRCOUT,
where V OUT?ESR is the voltage deviation of V OUT due to the
effects of ESR and the ESR COUT is the total effective series
resistance of the output capacitors.
A minimum capacitor value is required to sustain the
current during the load transient without discharging it. The
where Ipk?pk LOUT is the peak to peak current of the output.
From this equation it is clear that the ripple current increases
as L OUT decreases, emphasizing the trade?off between
dynamic response and ripple current.
Feedback and Compensation
The NCP1580 allows the output of the DC?DC converter
to be adjusted from 0.8 V to 5.0 V via an external resistor
divider network. The controller will try to maintain 0.8 V at
the feedback pin. Thus, if a resistor divider circuit was
placed across the feedback pin to V OUT , the controller will
regulate the output voltage proportional to the resistor
divider network in order to maintain 0.8 V at the FB pin.
voltage drop due to output capacitor discharge is given by
the following equation:
http://onsemi.com
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