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HFC0300 Datasheet(PDF) 9 Page - Monolithic Power Systems

No. de Pieza. HFC0300
Descripción  Variable Off Time Controller
Descarga  21 Pages
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Fabricante  MPS [Monolithic Power Systems]
Página de inicio  http://www.monolithicpower.com
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HFC0300 Datasheet(HTML) 9 Page - Monolithic Power Systems

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HFC0300– VARIABLE OFF TIME CONTROLLER
HFC0300 Rev. 1.0
www.MonolithicPower.com
9
9/23/2011
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2011 MPS. All Rights Reserved.
OPERATION
The HFC0300 incorporates all the necessary
features to build a reliable Switched-Mode Power
Supply (SMPS). Its high level of integration
requires few external components. Based on a
fixed peak current technique, the controller
decreases its frequency with the decreasing load
to minimize switching loss. When the output
power falls below a given level, the controller
enters burst mode. It also has better EMI
performance because the switching frequency
varies with the natural bulk ripple voltage.
Frequency Foldback
A capacitor connected to the FSET pin sets the
frequency at the end of charging. This capacitor
charges from a constant current source and its
voltage is compared with an internal threshold
fixed by COMP voltage (see Figure 2). When this
capacitor
voltage
reaches
threshold,
the
capacitor discharges rapidly down to 0V, and a
new period starts after a 0.6μs delay (see Figure
3).
S
R
_
Q
Q
28µA
3.3V
VOFFSET
0.88V
0.6µs
pulse
Drive
FSET
COMP
VCC
Figure 2: Voltage-Controlled Oscillation
Maximum Frequency
Minimum Frequency
IFSET=28µA
Vfset
Controlled by the
COMP Voltage
Pout Decrease
Pout Increase
Figure 3: COMP-Voltage–Adjusted Switching
Frequency
Start-up and Under Voltage Lock-out
Initially, the internal high voltage current source
drawn from the high-voltage (HV) pin powers the
IC. The IC starts switching and the internal high-
voltage current source turns off as soon as the
voltage on VCC reaches 11.7V. Then the
auxiliary winding of the transformer supplies the
IC before the VCC voltage falls back below 8.2V.
Otherwise, the switching pulse stops and the
high-voltage current source turns on again.
Figure 4 shows the typical waveform with VCC
under-voltage lockout (UVLO).
VCCH=11.7V
VCCL=8.2V
VCC
Internal
Current
Source
Driving
Signal
ON
OFF
The auxiliary
winding takes over
Figure 4: VCC Under-Voltage Lockout
The lower threshold of VCC UVLO goes from 8.2V
to 5.5V when fault conditions happen, such as
over-load protection (OLP), over-voltage protection
(OVP), and over-temperature protection (OTP).
Over-Voltage Protection
By monitoring the VCC pin with a 20µs time-
constant filter, the HFC0300 goes into latched
fault
condition
whenever
an
over-voltage
condition occurs—if VCC goes above 24V,
typically. The controller stays fully latched in this
position until the VCC is cycled down to 3.0V, e.g.
when the user unplugs the power supply from the
main input and re-plugs it.
Over-Load Protection
In a flyback converter, the maximum output
power is limited by the maximum switching
frequency and primary peak current. As the
primary peak current is constant, the maximum
power is limited by maximum frequency. When
the switching frequency reaches the maximum,


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