ML4818 Просмотр технического описания (PDF) - Fairchild Semiconductor
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ML4818 Datasheet PDF : 13 Pages
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ML4818
PRODUCT SPECIFICATION
The ML4818 can also be used in current mode by sensing
load current on the RAMP input (pin 3).
The four output delay timers are programmed via an external
RDELAY resistor as shown below. This resistor value should be
no less than 1kΩ. Expressing RDELAY in kΩ the delay, in ns
is:
TDELAY = 〈 33 × RDELAY〉 + 45
(1)
ISET
11
RT
1.7V
Q1
2
CT
5.5mA
5V
ISET
+
–
5V
CLOCK
OUT
13
250Ω
The ML4818 contains special logic circuits to provide for
voltage mode feed-forward and lock out long pulses into the
internal logic. This prevents instability from occuring when
the Φ Comparator trips in voltage mode.
Φ MOD
OUTPUT S
Q
OSC
R
RAMP
3
QR
Figure 3. Voltage Feed-Forward Circuit
The collector of QR in figure 3 is high only during a power
cycle. When the power cycle terminates, RAMP is pulled
low. In voltage mode operation, a capacitor is connected
from RAMP to GND with a resistor from RAMP to VIN to
provide input voltage feed forward.
Oscillator
The ML4818 oscillator charges the external capacitor, CT,
with a current (ISET) equal to 5/RT. When the CT voltage
reaches the upper threshold (Ramp Peak), the comparator
changes state, turning on the current sink which discharges
CT to the lower threshold (Ramp Valley). The CT pin is
clamped to Ramp Valley by Q1 (Figure 5) to prevent inaccu-
racy due to undershoot on CT.
To use the CLOCK output for driving external synchroniza-
tion circuitry, a pull-down resistor is required from CLOCK
to GND.
CLOCK
Figure 5. Ocillator Block Diagram
For frequencies of less than 500kHz, oscillator frequency
can be set by using the following formulae:
fOSC = -0---.--5---2----C----T---R-----1T----+-----5---0----0----C----T-
(2)
Error Amplifier
The ML4818 error amplifier is a 2.5MHz bandwidth, 8.5V/
µs slew rate op-amp with provision for limiting the positive
output voltage swing (output inhibit line) to implement the
soft start function. The error amplifier output source current
is limited to 4.5mA.
120
180
100
GAIN
135
80
PHASE
60
90
40
45
20
0
0
0
100
1k
10k
100k
1M
10M
FREQUENCY
Figure 6. Error Amplifier Open-Loop Gain and Phase vs.
Frequency
VCC
VCC
RAMP PEAK
CT
RAMP VALLEY
TC
TD
Figure 4. Ocillator Timing Diagram
Q2
OUT
Q1
POWER
GND
Figure 7. Power Driver Simplified Schematic
7
REV. 1.0.3 6/21/01
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