FAN8048 データシートの表示（PDF） - Fairchild Semiconductor
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FAN8048 Datasheet PDF : 34 Pages
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FAN8048
3-3. Oscillator
Oscillator frequency is determined by the charging/discharging current iCG and iDCG of the internal circuit and capacitor
(CPCT) connected to PCT (pin7) and ground. To change oscillator frequency, you may change the CPCT capacitor.
For example, the external capacitor (CPCT) value can be calculated as follows:
t PCT
= CPCT × ∆VPCT
iCG
+ CPCT × ∆VPCT
iDCG
= CPCT × ∆VPCT(iCHG + iDCG )
iCG × iDCG
f PCT
=1
t PCT
=
CPCT
×
iCG × iDCG
∆VPCT ×(iCG
+
iDCG
)
CPCT
=
iCG × iDCG
f PCT × ∆VPCT ×(iCG + iDCG )
Where, iCG is charging current, which is 42uA, iDCG is discharging current, which is 14uA, and oscillator peak-peak voltage,
∆VPCT is approximately 300mV. This oscillator is designed to synchronize the frequency of the oscillator itself to the clock
pulse frequency separately input to external CLK (pin8). To utilize this function, the oscillator frequency itself should be con-
figured lower than the frequency of the external synchronous signal.
3-4. Error Amplifier
The error amplifier of the DC-DC converter is used to amplify the difference between internal reference voltage and output
voltage. This amplified voltage generates a square wave pulse corresponding to the difference of triangular waveform-PCT
output formed by triangular wave oscillatory circuit of pulse width modulation comparator (PWM comparator), whereby the
square wave pulse stabilizes the output voltage by operating the DC-DC converter's switching devices through the operation
circuit. The most well-known system stabilization method using an error amplifier is pole-zero compensation. Detailed system
design standards and methods will be discussed in a later section of this document.
3-5. Short Circuit Protection Function
The short circuit protection is a function to protect circuits from being damaged from various abnormal conditions such as over
current or circuit shorts; and on this occasion, when the error amplifier output voltage, EIO1, (pin10) of DC-DC converter1
reaches the specific voltage (typically 1.35V), the internal current source, iSPRT, start charging the external capacitor, CSPRT
connected between SPRT(pin5) and ground as shown in Figure 1 and the DC-DC converter circuit will be shutdown. Also, to
prevent malfunction, this function activates only when the circuit short condition exists for a certain amount of time. This time
setting (TSPRT) is determined according to the capacitance of external capacitor CSPRT, and its formula is as follows:
TSPRT
=
CSPRT × 0.5 [sec]
iSPRT
Where, iSPRT is charging current, which is 6uA.
3-6. Soft Start
This function limits overshoot in the initial operation. This circuit operates when DC-DC converter 1 output voltage rises over
a specific voltage (typically 1.48V), thereafter it starts charging the external capacitor CSOFT connected between SOFT (pin9)
and Ground. It restricts the error amplifier output voltage caused by sharp-rising capacitor voltage. Soft start time is deter-
mined by the following formula:
When the output voltage of the conveter, VSYS1, is brought above typically 1.48V, the soft start function is enable and the
internal current source is begin to charging the capacitor, CSOFT. A detailed diagram of this fuction is shown in Figure 8. The
component CSOFT provide a slow ramping voltage on the SOFT pin to provide a soft start function. The time constant in this
case is shown by the next formular.
TSOFT = CSOFT × iSOFT = CSOFT ×10uA[sec]
where, iSOFT = 10uA
15
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