ML4826 Просмотр технического описания (PDF) - Micro Linear Corporation
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ML4826 Datasheet PDF : 16 Pages
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FUNCTIONAL DESCRIPTION (Continued)
Volts peak. The composite waveform feeding the RAMP2
pin for the PWM consists of the reflected output current
signal along with the transformer magnetizing current and
the slope compensation signal.
Equation 8 describes the composite signal feeding
RAMP2, consisting of the primary current of the main
transformer and the slope compensation. Equation 9
solves for the required slope compensation peak voltage.
VRAMP2
=
IPRI +
1×
2
VOUT
L
× NS
NP
×
TS
×
1
nCT
≤ VFB
– 1.5V
(8)
VSC
=
21 ×
VOUT
L
× NS
NP
×
TS
×
RSENSE
nCT
=
1×
2
48V ×
20µH
14
90
× 5µ sec 471Ω
200
= 2.2V
(9)
Soft Start
Start-up of the PWM is controlled by the selection of the
external capacitor at SS. A current source of 50µA supplies
the charging current for the capacitor, and start-up of the
PWM begins at 1.5V. Start-up delay can be programmed
by the following equation:
CSS
=
tDELAY
×
50µA
1.5V
(10)
where CSS is the required soft start capacitance, and
tDELAY is the desired start-up delay.
ML4826
It is important that the time constant of the PWM soft-start
allow the PFC time to generate sufficient output power for
the PWM section. The PWM start-up delay should be at
least 5ms.
Solving for the minimum value of CSS:
CSS
=
5ms ×
50µA
1.5V
= 167nF
(11)
VCC
The ML4826 is a current-fed part. It has an internal shunt
voltage regulator, which is designed to regulate the
voltage internal to the part at 13.5V. This allows a low
power dissipation while at the same time delivering 10V
of gate drive at the PWM OUT and PFC OUT outputs. It is
important to limit the current through the part to avoid
overheating or destroying the part.
There are a number of different ways to supply VCC to the
ML2826. The method suggested in Figure 5, is one which
keeps the ML4826 ICC current to a minimum, and allows
for a loosely regulated bootstrap winding. By feeding
external gate drive components from the base of Q1, the
constant current source does not have to account for
variations in the gate drive current. This helps to keep the
maximum ICC of the ML4826 to a minimum. Also, the
current available to charge the bootstrap capacitor from
the bootstrap winding is not limited by the constant
ISENSE
x Former
4 x IN4148
T3
200:1
Q14
2N2222
D1
C26
220pF
R16
471Ω
R21
8.63kΩ
C11
1000pF
17 VCC
18 VREF
7 RTCT
R40
47.0kΩ
RAMP2
9
1.5V
AGND
11
1V
R13
2.2kΩ
10 DC ILIMIT
R38
10.0kΩ
U2
6 VDC
PWM CMP
–
+
DC ILIMIT
–
+
Figure 4. Slope Compensation and Current Sense
11
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