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参数资料
| 型号： | PCZ34670EG |
| 厂商： | FREESCALE SEMICONDUCTOR INC |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING REGULATOR, 400 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封装： | 1.27 PITCH, ROHS COMPLIANT, PLASTIC, MS-013AC, SOIC-20 |
| 文件页数： | 7/24页 |
| 文件大小： | 986K |
| 代理商： | PCZ34670EG |
Analog Integrated Circuit Device Data
Freescale Semiconductor
15
34670
FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
converters running in continuos conduction mode (CCM).
The value of the slope compensation depends on the
switching frequency. See Table 10.
ISOLATED OPTOCOUPLER FEEDBACK
Isolated voltage feedback can be accomplished by using
an optocoupler and a shunt regulator (see Figure 19). The
output voltage accuracy is a function of the accuracy of the
shunt regulator and feedback resistor divider tolerance,
therefore the feedback resistors should have an appropriate
accuracy.
Since the error amplifier function is implemented on the
secondary side by the optocoupler and a 3-pin adjustable
shunt regulator, the internal error amplifier of the 34670 is not
used. The FB pin is connected to VOUT, thus disabling the
internal open-drain error amplifier.
The bias voltage for the optocoupler is accomplished
through the internal 5.0 k
pull-up resistor between COMP
and an internal 5.0 V reference.
When a TL431 or TLV431 shunt regulator is used for
output voltage regulation, the output voltage is set by the ratio
voltage is given by the following equation:
where VREF = 1.24 V for the TLV431 (VREF = 2.5 V for the
TL431).
Figure 16. Isolated Optocoupler Feedback
ISOLATED PRIMARY CONTROL FEEDBACK
Another option to accomplish isolated feedback is the use
of a tertiary winding (see Figure 21). The advantage of this
solution without optocoupler and shunt regulator is clearly the
cost effectiveness. Nevertheless the line and load regulation
is worse than with optocoupler feedback.
When isolated primary feedback is used, the loop
compensation components are connected between pins
COMP and FB.
INTERNAL REGULATORS
The internal high voltage regulator of the 34670 regulates
from the input voltage across VPWR and VIN down to the
VDD voltage. During start-up the high voltage regulator
provides the necessary voltage for the internal gate driver to
commence switching. If the external MOSFET gate drive
pulls less than 3.0 mA under all circumstances, an auxiliary
transformer winding that usually provides the bias voltage for
the chip and the gate driver is not required.
In cases where the external MOSFET gate drive pulls
more than 5.0 mA, an auxiliary winding is needed to reduce
the power dissipation in the internal high voltage LDO. See
Figure 18 for an application drawing. It is recommended to
add a 0.1
F ceramic capacitor in parallel with the existing
load capacitor. This reduces noise at the VDD pin caused by
the auxiliary winding.
The high voltage regulator is disabled when the VDD pin is
forced by an external voltage above the VDD regulation point.
Table 10. Slope Compensation Values
Switching Frequency [kHz]
Slope Compensation [mV/
s]
100
10
250
25
400
50
V
O
V
REF
1
R
1
R
2
-------
+
=
T1
NP
NS
RV
R1
TLV431
R2
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