AN1025 데이터 시트보기 (PDF) - Microchip Technology
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AN1025 Datasheet PDF : 8 Pages
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AN1025
Designing with a Charge Pump
Output voltage ripple and charge pump strength are
affected by the style and value of the capacitors used.
Typically, low ESR capacitors should be used for the
input and output capacitors. This helps minimize noise
and ripple in the system.
The value of the input capacitor is somewhat dictated
by the system voltage supply. If the source impedance
to the charge pump is very low, the input capacitor
might not be needed. However, if there is a large
source impedance, an input capacitor is needed to help
prevent ripple on the input voltage pin.
Output voltage ripple is controlled by the amount of
capacitance in the output capacitor. Large values of
output capacitance will reduce the output ripple at the
expense of a slower turn-on time from shutdown and a
higher in-rush current.
The fly capacitor controls the strength of the charge
pump. However, care must be taken when selecting the
value of this capacitor. Recall that the maximum charge
time for the fly capacitor is one half the charge pump
oscillator frequency and when charging, it is in series
with the ON resistance of two switches. The charging
time constant of this RC circuit should be less than the
maximum charge time.
BUCK SWITCHING REGULATOR
One of the simplest switch mode converters is the buck
converter. The buck converter is an inductor-based
converter used to step-down an input voltage to a lower
magnitude output voltage. It is similar to the LDO circuit
previously discussed, but with one main difference.
Instead of the pass transistor that functions as a
variable resistor in the LDO, the MOSFET in a buck
converter is either ON or OFF. The regulation of the
output voltage is achieved by controlling the ON and
OFF time of this MOSFET. This allows the buck
regulator to convert a high source voltage to a
regulated lower output voltage efficiently.
Buck Converter Operation
A basic buck regulator schematic is shown in Figure 4.
A typical buck regulator consist of a switching
MOSFET, an inductor, output capacitor and a
recirculating diode. During a switching cycle, the
MOSFET, Q1, transitions between an ON state and an
OFF state. Assume the buck regulator is operating in
steady-state and Q1 is in the ON state. The voltage
across the inductor, L1, is equal to the input voltage,
VIN, minus the output voltage, VOUT. Energy is being
stored in L1. At the end of the ON time, tON, Q1
transitions to an OFF state. The voltage across L1
collapses, changing polarity to a value equal to -VOUT.
The energy in L1 is now decreasing and suppling the
output requirements. Q1 remains OFF until the end of
the period. This complete cycle is then repeated.
DS01025A-page 4
Q1
VOUT
VIN
CIN
L1
D1 COUT
RL
FIGURE 4:
Schematic.
Buck Regulator System
Understanding the operation of the buck converter and
realizing that the volt-time across the inductor in the ON
time must equal the inductor volt-time in the OFF time
allows a relationship between the input voltage and
output voltage to be established. This input to output
voltage relationship is shown in Equation 2.
EQUATION 2:
Where:
DutyCycle = V----VO----I-U-N----T--
Duty Cycle = tON / (tON + tOFF)
Synchronous Buck Converters
When a buck converter is used to generate low output
voltages, the recirculating diode, D1 in Figure 4, can be
replaced with another MOSFET and is switched out-of-
phase with the main MOSFET. By doing so, the overall
system efficiency is improved. For example, a buck
converter is used to generate an output voltage of 3.0V
and D1 has a forward voltage drop, VFD, of 0.75V.
There would be approximately an initial 25% decrease
in the buck converters maximum efficiency because of
the diode’s VFD. The efficiency degradation would be
worse with a lower output voltage.
Microchip offers a number of synchronous buck
converter regulators. Devices like the MCP1601 or
MCP1612 integrate both the main switching MOSFET
and the synchronous MOSFET. Figure 5 shows an
adjustable output voltage, synchronous buck converter.
The items in the dashed box are contained within the
buck IC. Another Microchip device, the TC1303,
contains both a synchronous buck regulator with
integrated MOSFETs and an LDO.
© 2006 Microchip Technology Inc.
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