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| 型号： | NCP1575DG |
| 厂商： | ON Semiconductor |
| 文件页数： | 11/16页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM 8-SOIC |
| 产品变化通告： | Product Obsolescence 08/Apr/2011 |
| 标准包装： | 98 |
| PWM 型： | 电流/电压模式，V²? |
| 输出数： | 1 |
| 频率 - 最大： | 320kHz |
| 电源电压： | 9 V ~ 20 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： | 0°C ~ 125°C |
| 封装/外壳： | 8-SOIC（0.154"，3.90mm 宽） |
| 包装： | 管件 |
NCP1575
8.5 V
V IN
to an overvoltage condition within 200 ns, turning off the
upper MOSFET and disconnecting the regulator from its
input voltage. This results in a crowbar action to clamp the
0.465 V
V COMP
output voltage, preventing damage to the load. The regulator
remains in this state until the overvoltage condition ceases.
V FB
Shutdown
When the input voltage connected to V CC falls through the
GATE(H)
lower threshold of the UVLO comparator, a fault latch is set.
The fault latch provides a signal that forces both GATE(H)
UVLO
STARTUP
t S
NORMAL OPERATION
low and GATE(L) high, producing a low?impedance current
sink to ground at the converter switch node. At the same
Figure 21. Idealized Waveforms
Normal Operation
During normal operation, the duty cycle of the gate drivers
remains approximately constant as the V 2 control loop
maintains the regulated output voltage under steady state
conditions. Variations in supply line or output load conditions
will result in changes in duty cycle to maintain regulation.
Input Supplies
The NCP1575 can be used in applications where a 12 V
supply is available along with a lower voltage supply. Often
the lower voltage supply is 5 V, but it can be any voltage less
than the 12 V supply minus the required gate drive voltage
of the top MOSFET. The greater the difference between the
two voltages, the better the efficiency due to increasing V GS
available to turn on the upper MOSFET. In order to maintain
power supply stability, the lower supply voltage should be
at least 1.5 times the desired voltage.
Adding a few additional components allows the NCP1575
to convert power in a “12 V only” application. This circuit
is illustrated in Figure 1. Note that in all cases, the maximum
supply voltage specification of 20 V must not be exceeded.
Gate Charge Effect on Switching Times
When using the onboard gate drivers, the gate charge has
an important effect on the switching times of the FETs. A
finite amount of time is required to charge the effective
capacitor seen at the gate of the FET. Therefore, the rise and
fall times rise linearly with increased capacitive loading.
Transient Response
The 200 ns reaction time of the control loop provides fast
transient response to any variations in input voltage and
output current. Pulse?by?pulse adjustment of duty cycle is
provided to quickly ramp the inductor current to the required
level. Since the inductor current cannot be changed
instantaneously, regulation is maintained by the output
capacitors during the time required to slew the inductor
current. For better transient response, several high
frequency and bulk output capacitors are usually used.
Overvoltage Protection
Overvoltage protection is provided as a result of the
normal operation of the V 2 control method and requires no
additional external components. The control loop responds
time, the latch also turns on a transistor which pulls down on
the COMP pin, quickly discharging the external capacitor,
and allowing COMP to fall.
CONVERTER DESIGN
Choosing the V OUT Resistor Divider Values
The NCP1575 has an internal 0.98 V reference. A resistor
divider is used to set the output voltage.
V OUT
R1
V FB
R2
Figure 22.
The formula to set the output voltage is
VOUT + (R1 R2 ) 1) < (0.98 V)
Arbitrarily choose a value of R2 that is sufficiently low
that the V FB bias current (typically 50 nA) will have
negligible effect on the output voltage. Solve the equation
above for the value of R1.
Choosing the Oscillator Frequency
The NCP1575 has an oscillator that is trimmed to 200 kHz
at the factory. The NCP1575 will operate at this frequency
without the addition of any external components. However,
the oscillator is user?programmable with a single resistor.
This resistor is connected between the R OSC pin and ground.
Adding this resistor will raise the frequency above 200 kHz.
A graph of oscillator frequency vs. R OSC resistance is
provided in the typical operating characteristics section of
this data sheet.
Selection of the Output Capacitors
These components must be selected and placed carefully
to yield optimal results. Capacitors should be chosen to
provide acceptable ripple on the regulator output voltage.
Key specifications for output capacitors are their Equivalent
http://onsemi.com
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