TK11830
CIRCUIT OPERATION (CONT.)
Oscillator Frequency:
Where:
f = 1/(tON + tOFF)
And:
tON = L x [ILPK / (VIN - VSAT)]
tOFF = L x [ILPK / (|VOUT| + VF)]
Therefore:
f=
1
ILPKL × VIN
1
− VSAT
+
1
VOUT + VF
The ESR of the capacitor and the effect of the input voltage
difference for the comparator function are added to VRIPPLE.
The maximum inductor current is limited by the power
transistor switch capacity: ILPK(MAX) ~ 300 mA.
Output Voltage is as follows:
VOUT = (Vref / 5) x (1 - 4 x R2 / R1)
where: Vref = 1.28 V
R3, R4: IC Internal
R4 / R3 = 1 / 4
R1, R2 : External Resistor
Vref
R3
R1
( ( ) ( ) ) =
VIN − VSAT 2 VOUT + VF
2ILOAD VIN − VSAT + VOUT + VF
2
×1
L
R4
R2
VOUT
tON / tOFF
(|VOUT| + VF) / (VIN - VSAT)
ILPK
2 x ILOAD x [(tON / tOFF) + 1]
f
( ) ( ) VIN − VSAT 2 VOUT + VF
( ) 2ILOAD VIN − VSAT + VOUT + VF
2
•1
L
COUT
(ILOAD x tON) / VRIPPLE
January 1999 TOKO, Inc.
Page 9