FA7700V Datasheet, PDF (11/13 Pages) List of Unclassifed Manufacturers – For Switching Power Supply Control

FA7700V, FA7701V
You can reset the off latch mode operation of the short-circuit
protection by either of the following ways: lowering the CS
voltage below 2.03V (typ.); lowering the Vcc voltage below the
Off threshold voltage of undervoltage lock out; 1.93V (typ.);
lowering the voltage of FB terminal below 1.5V (typ.)
The off latch mode action cannot be triggered by externally
applying voltage of over 2.2V forcibly to the CS terminal (1.5V,
ZD clamped). Characteristics of the current and the voltage of
CS terminal is shown in the characteristic curve (CS terminal
voltage vs. CS terminal sink current) on page 6. Be sure to use
the IC up to the recommended CS terminal current of 50µA.
8. Output circuit
The IC contains a push-pull output stage and can directly drive
MOSFETs (FA7700: N ch, FA7701: P ch). The maximum peak
current of the output stage is a sink current of +150mA, and a
source current of –400mA. The IC can also drive NPN, and
PNP transistors. The maximum peak current in such cases is
±50mA. Be sure to design the output current considering the
rating of power dissipation.
9. Power good signal circuit/ Undervoltage lockout circuit
The IC contains a protection circuit against undervoltage
malfunctions to protect the circuit from the damage caused by
malfunctions when the supply voltage drops. When the supply
voltage rises from 0V, the circuit starts to operate at VCC of
2.07V (typ.) and outputs generate pulses. If a drop of the
supply voltage occurs, it stops output at VCC of 1.93V (typ.).
when it occurs, the CS terminal is turned to Low level and then
it is reset. The power good signal circuit monitors the voltage of
REF terminal, and stops output until the voltage of REF
terminal excesses approximately 2V to prevent malfunctions.
s Design advice
1. Setting the oscillation frequency
As described in item 2 “Oscillator” of “Description of each
circuit”, a desired oscillation frequency can be determined by
the value of the resistor connected to the RT terminal. When
designing an oscillation frequency, you can set any frequency
between 50kHz and 1MHz. You can roughly obtain the
oscillation frequency from the characteristic curve “Oscillation
frequency (fosc) vs. timing resistor resistance(RT)” or the value
can be calculated by the following expression.
fOSC = 3000 ϫ RT ^–0.9
( ) RT = 3000 1.11
fOSC
fOSC: Oscillation frequency [kHz]
RT: Timing resistor [kΩ]
This expression, however, can be used for rough calculation,
the value obtained is not guaranteed. The operation frequency
varies due to the conditions such as tolerance of the
characteristics of the ICs, influence of noises, or external
discrete components. When determining the values, be sure to
verify the effectiveness of the values of the components in an
actual circuit.
2. Operation around the maximum or the minimum output
duties
As described in characteristic curves on page 5, “output duty
cycle vs. FB terminal voltage (VFB)” and “output duty cycle vs.
CS terminal voltage (Vcs)”, the linearity of the output duty of
this IC drops around the minimum output duty and the
maximum output duty (FA7701 only). This phenomena are
conspicuous when operating in a high frequency (when the
pulse width is narrow). Therefore be careful when using high
frequency.
3. Restriction of external discrete components
To achieve a stable operation of the ICs, the value of external
discrete components connected to Vcc, REF, CS, FB terminals
should be within the recommended operational conditions.
4. Loss calculation
Since it is difficult to measure IC loss directly, the calculation to
obtain the approximate loss of the IC connected directly to a
MOSFET is described below.
When the supply voltage is Vcc, the current consumption of the
IC is Icc, the total input gate charge of the driven MOSFET is
Qg, the switching frequency is fsw, the total loss Pd of the IC
can be calculated by:
Pd Ô¼ Vcc Ï« (Icc + Qg Ï« fsw).
The values in this expression is influenced by the effects of the
dependency of supply voltage, the characteristics of
temperature, or tolerance. Therefore, be sure to verify
appropriateness of the value considering the factors above
under all applicable conditions.
Example:
When VCC = 6V, in the case of a typical IC, from the
characteristic curve, Icc=1.2mA. When operating in Qg = 6nC,
fsw = 500kHz, Pd should be:
Pd Ô¼ 6 Ï« (1.2mA + 6nC Ï« 500kHz) Ô¼ 25.2mW
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