MB3769A Datasheet, PDF (16/26 Pages) Fujitsu Component Limited. – SWITCHING REGULATOR CONTROLLER

MB3769A
s HOW TO SYNCHRONIZE WITH OUTSIDE CLOCK
The MB3769A oscillator circuit is shown in Figure 16. CT charge and discharge currents are expressed by the following formula:
ICT =
±2 x I1
=±
5V
RT
VREF
Fig. 16 -Oscillator Circuit
1 kΩ
500
Ω
I1
2 x I1
RT
-
6
+
2.5 V
300
Ω
500 Ω
+
2 x I1
ICT
CT
5
(4 x I1)
-
3.5 V
-
+
1.5 V
150Ω
S
Q
R
This circuit shows that if the voltage at the CT terminal is set to 1.5 V or less, one oscillation cycle ends and the next cycle starts.
An example of an external synchronous clock circuit is shown in Figure 17.
Fig. 17 -Typical Connection of Synchronized Outside Clock Circuit
tcycle
5
MB3769A
6
RT ^CT
ex. MB74HC04
VP
R(5.1 k Ω)
clamp circuit
(VL)
VP
tP
tcycle = 2.5 µs (fEXT = 400 kHz)
tP = 0.5 µs
RT = 11 k Ω
The Figure 18 shows the CT terminal waveform.
VTH may be near 2.5 V. In this case, the maximum duty cycle is restricted
as shown in the formula below if tP’ = 0.
Dmax= ^{(3.5 - 1.85) + (3.5 - VTH)} ≤ 59% (VL = 0 V: No clamp circuit)
(3.5 - VL ) + (3.5 - VTH )
When VTH = 2.5 V, CT can be provided by followings.
Fig. 18 -Voltage Waveform at CT
1.85
VCT
3.5 V
VTH
(.. 2.5 V)
VL
tP’
tcycle - tP =
1
x
fOSC
(3.5 - VL) + (3.5 - VTH)
fOSC(3.5 - 1.5 ) x 2
16