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| 型号： | PC34703PNB/R2 |
| 厂商： | FREESCALE SEMICONDUCTOR INC |
| 元件分类： | 稳压器 |
| 英文描述： | 18 A SWITCHING REGULATOR, 400 kHz SWITCHING FREQ-MAX, PQCC33 |
| 封装： | 10 X 10 MM, 2.1 MM HEIGHT, 0.8 PITCH, PLASTIC, QFN-33 |
| 文件页数： | 14/46页 |
| 文件大小： | 2509K |
| 代理商： | PC34703PNB/R2 |
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Analog Integrated Circuit Device Data
Freescale Semiconductor
21
34703
FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
Figure 15. LDO Converter Overcurrent Protection
Switching Oscillator
A 300 kHz (default) oscillator sets the switching frequency
of the buck regulator. The frequency of the oscillator can be
adjusted between 200 kHz and 400 kHz by an optional
external resistor RF connected from the FREQ terminal of the
integrated circuit to ground. See Figure 10 for frequency
resistor selection.
The CLKSYN terminal can be configured either as an
oscillator output when the CLKSEL terminal is left open or it
can be used as a synchronization input when the CLKSEL
terminal is grounded. The oscillator output signal is a square
wave logic signal with 50 percent duty cycle, 180 degrees
out-of-phase with the internal clock signal. This allows
opposite phase synchronization of two 3370x devices.
When the CLKSYN terminal is used as synchronization
input (CLKSEL terminal grounded), the external resistor RF
chosen from the chart in Figure 10 should be used to
synchronize the internal ramp generator to the external clock.
Operation is only recommended between 200 kHz and
400 kHz. The supplied synchronization signal does not need
to be 50 percent duty cycle. Minimum pulse width is 300 ns.
Low Dropout Linear Regulator (LDO)
The adjustable low dropout linear regulator (LDO) is
capable of supplying a 2.0 A output current. It has a current
limit with retry capability.
Current limit monitoring can be used in 2 ways: either
using a sense resistor or directly tying the ISNS terminal to
the booster voltage VBST. With the sense resistor used, the
current limit window is 10 ms. In the other configuration, the
current limit window is 1.0 ms.
When the sense resistor is used, the control circuit limits
the current for 10 ms when the voltage measured across the
current sense resistor reaches the 45 mV threshold. If the
overcurrent condition still exists, the linear regulator is turned
off. At the same time the overcurrent condition is detected,
the Retry Timer starts to time out. When the timer expires
after 100 ms, the LDO tries to power up again for 10 ms,
repeatedly checking for the overcurrent condition. The
current limit of the LDO can be set by using the following
formula:
ILIM = 45 mV/RS
Where RS is the LDO current sense resistor, connected
between the ISNS terminal and the LDO terminal output (see
When the sense resistor is not used, the overcurrent
condition is sensed by saturation of the linear regulator driver
buffer.
The output voltage of the LDO can be adjusted by means
of an external resistor divider connected to the feedback
control terminal LFB. The linear regulator output voltage can
be adjusted in the range of 0.8 V to 5.0 V, but the LDO output
voltage is always lower than the input voltage to the regulator.
Power-up, power-down, and fault management are
coordinated with the switching regulator.
Thermal Shutdown
In order to increase the overall safety of the system
designed with the 34703, an internal thermal shutdown
function has been incorporated into the switching regulator
circuit. The 34703 senses the temperature of the buck
regulator main switching FET (high-side FET Q1; see
Figure 2), the low-side (synchronous FET Q2), and control
circuit. If the temperature of any of the monitored components
exceeds the limit of safe operation (thermal shutdown), the
switching regulator will be shut down. After the temperature
falls below the value given by the thermal shutdown
hysteresis window, the switcher will retry to operate again.
The VOUT/LDO pull-down FETs Q3/Q4 have an
independent thermal shutdown control. When the Q3/Q4
temperature exceeds the thermal shutdown limit, Q3/Q4 will
be turned off without affecting the switcher operation.
The maximum junction temperature is 125°C and thermal
shutdown is 170°C. It is not recommended to operate this IC
beyond these thresholds.
Watchdog Timer
A watchdog function is available via I2C bus
communication. It is possible to select either window
watchdog or time-out watchdog operation, as illustrated in
Watchdog time-out starts when the watchdog function is
activated via I2C bus sending a Watchdog Programming
command byte, thus determining watchdog operation
(window or time-out) and period duration (refer to Table 6). If
the watchdog is cleared by receiving a new Watchdog
Programming command through the I2C bus, the watchdog
timer is reset and the new time-out period begins. If the
watchdog time expires, the RESET will become active (LOW)
for a time determined by the RC components of the RT timer
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