ML4800 View Datasheet(PDF) - Fairchild Semiconductor
Part Name
Description
Manufacturer
ML4800 Datasheet PDF : 14 Pages
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PRODUCT SPECIFICATION
ML4800
where CSS is the required soft start capacitance, and tDELAY
is the desired start-up delay.
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 × 1-2---.5-2--µ-5---AV--- = 100nF
(6a)
Caution should be exercised when using this minimum soft
start capacitance value because premature charging of the SS
capacitor and activation of the PWM section can result if
VFB is in the hysteresis band of the VIN OK comparator at
start-up. The magnitude of VFB at start-up is related both to
line voltage and nominal PFC output voltage. Typically, a
1.0µF soft start capacitor will allow time for VFB and PFC
out to reach their nominal values prior to activation of the
PWM section at line voltages between 90Vrms and
265Vrms.
Generating VCC
The ML4800 is a voltage-fed part. It requires an external
15V, ±10% (or better) shunt voltage regulator, or some other
VCC regulator, to regulate the voltage supplied to the part at
15V nominal. This allows low power dissipation while at the
same time delivering 13V nominal gate drive at the PWM
OUT and PFC OUT outputs. If using a Zener diode for this
function, it is important to limit the current through the
Zener to avoid overheating or destroying it. This can be
easily done with a single resistor in series with the Vcc pin,
returned to a bias supply of typically 18V to 20V. The
resistor’s value must be chosen to meet the operating current
requirement of the ML4800 itself (8.5mA, max.) plus the
current required by the two gate driver outputs.
EXAMPLE:
With a VBIAS of 20V, a VCC of 15V and the ML4800 driving
a total gate charge of 90nC at 100kHz (e.g., 1 IRF840
MOSFET and 2 IRF820 MOSFETs), the gate driver current
required is:
IGATEDRIVE = 100kHz × 90nC = 9mA
(7)
RBIAS
=
V-----B----I--A---S-----–----V-----C---C--
ICC + IG + IZ
(8)
RBIAS
=
------------2---0----V------–----1---5---V---------------
6mA + 9mA + 5mA
=
250Ω
Choose RBIAS = 240Ω.
The ML4800 should be locally bypassed with a 1.0µF
ceramic capacitor. In most applications, an electrolytic
capacitor of between 47µF and 220µF is also required across
the part, both for filtering and as part of the start-up bootstrap
circuitry.
L1
I1
+
VIN
DC
SW2 I2 I3
I4
SW1
RL
C1
REF +–EAU3
RAMP
OSC
CLK
U4
+
– U1
DFF
RQ
D U2
Q
CLK
RAMP
VEAO
VSW1
TIME
TIME
Figure 4. Typical Trailing Edge Control Scheme
REV. 1.0.5 9/25/01
11
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