AN968

AN968
Simple Synchronous Buck Converter Design - MCP1612
Author:
Cliff Ellison
Microchip Technology Inc.
Solving the standard inductor equation for inductor
ripple current (∆IL) yields:
VL
∆I L = ------ × ∆T
L
INTRODUCTION
The density of portable electronic equipment requires
the design engineer to pay particular attention to a
number of important design parameters. For the power
conversion circuitry, two of these parameters are the
efficiency and the total circuitry footprint. Keeping the
efficiency high extends battery life and controls the
temperature rise of the equipment. Limiting the circuitry
footprint helps minimize the size of the Printed Circuit
Board (PCB) and, ultimately, the total cost of the
device.
The MCP1612 is an ideal choice for such a design.
Since both the switching and synchronous MOSFETs
are internal and switching is 1.4 MHz, the inductor,
input and output capacitor size is minimized. The
output voltage is set by using a simple resistor divider,
with a high-bandwidth loop being accomplished by a
series resistor capacitor to ground. Efficiencies of 90%
and a typical shutdown current of 0.01 µA help to
extend battery life.
This Application Note contains all of the information
needed to design a synchronous buck converter using
the MCP1612. It also contains a real-world design
example with measured laboratory data.
POWER COMPONENT DESIGN
The design of the power components for the MCP1612
is made easier because the switching and synchronous
MOSFETs are internal. The output filter inductor and
capacitor are the only two power components that need
to be selected.
Where:
VL = voltage across the inductor (VIN – VOUT)
L
= value of inductance
∆T = on-time of switching MOSFET
When operating in Continuous Conduction mode
(meaning the inductor current never goes to zero), the
on-time (∆T) of the P-channel MOSFET is determined
by multiplying the duty cycle by the switching period.
Using the output voltage (VOUT) to input voltage (VIN)
relationship, the duty cycle yields:
V OUT
DutyCycle = ------------VIN
Therefore, it follows that ∆T is:
1
∆T = DutyCycle × ---------F SW
Where:
FSW = switching frequency
Most inductor manufacturers specify the peak current
that an inductor can support before the inductance
drops by a given percentage. ∆IL should be reevaluated if the inductance drops by a large percentage
because of its peak current. The peak inductor current
is calculated by:
1
IL ( PEAK ) = I OUT ( MAX ) +  --- × ∆I L
2

Buck Inductor
The inductance and current-carrying capability of the
buck inductor or output filter inductor is very easy and
straightforward to calculate. The size of the inductor is
selected such that a certain ripple current is achieved.
As will be shown later, the amount of allowable ripple
current determines the amount of output ripple voltage
present at the converter load.
 2005 Microchip Technology Inc.
DS00968A-page 1
AN968
1
VIN
LX
VIN
8
MCP1612
2
0.1 µF
3
ON
VOUT
COUT
10Ω
CIN
L
VCC
PGND
SHDN
AGND
RTOP
7
6
RBOT
OFF
4
Comp
FB
5
RC
CC
FIGURE 1:
Typical Application Schematic.
Output Capacitor
SETTING OUTPUT VOLTAGE
The MCP1612 is designed to allow the use of ceramic,
tantalum or aluminum electrolytic capacitors as output
filter capacitors. The output capacitor is chosen to meet
the output ripple specification and to provide storage
for load transients. The value of the capacitance is not
the only parameter of the capacitor that determines
ripple voltage. All capacitors have an Equivalent Series
Resistance (ESR) that contributes to the ripple voltage.
Ceramic capacitors have the lowest ESR, but increase
in cost with higher capacitance values. Aluminum electrolytic and tantalum capacitors are relatively inexpensive in higher capacitance values, but they also have a
much higher ESR.
The output voltage of the MCP1612 is easily set by the
use of an external resistor divider network. The divideddown output voltage is internally compared to a 0.8V
reference. The output voltage can be set to any voltage
between 0.8V and VIN.
Solving the standard capacitor equation for the output
ripple voltage (∆VC) yields:
IC × ∆T
∆VC = ------------------C OUT
Where:
IC
A resistor value of 200 kΩ or lower is recommended for
RBOT, the lower end of the resistor divider. If a highervalue resistor is used, the circuit will become more
susceptible to noise at the feedback pin. The top
resistor (RTOP) of the resistor divider is easily
calculated by using the following equation:
V OUT
RTOP = R BOT ×  ------------- – 1
 V FB

Where:
VOUT = desired output voltage
VFB
= MCP1612 internal reference voltage
RTOP = top resistor value
= peak-to-peak ripple current
∆T = on-time of P-channel MOSFET
RBOT = bottom resistor value
∆VC = output ripple voltage
As previously stated, the capacitor ESR also contributes to the output ripple voltage. This ripple voltage
(VESRRIPPLE) is defined as:
V ESRRIPPLE = I C × ESR
DS00968A-page 2
 2005 Microchip Technology Inc.
AN968
INPUT AND VCC FILTER
JUNCTION TEMPERATURE RISE
Input Capacitor
The input current in a buck topology is pulled from the
source and input capacitor in pulses. The size of the
input capacitor determines the amount of peak current
that is pulled from the source. The input capacitor also
reduces the amount of voltage ripple present at the
input to the converter.
The value of the input capacitor can be calculated the
same way as the output capacitor. A capacitance value
is chosen and the corresponding ripple voltage is
calculated. For most applications, a 10 µF ceramic
capacitor connected between the VIN and PGND pins of
the MCP1612 is recommended to filter the current
pulses. A lower-value capacitor can be used in applications that have a low source impedance. Ceramic or
aluminum electrolytic capacitors can be used, but the
capacitor ripple current rating should not be exceeded.
VCC Filter
The VCC pin provides bias for the internal analog
circuitry. It is important that this voltage stay free of line
transients and, therefore, it is recommended that a
separate filter be used for this voltage. Placing a 10Ω
resistor between VIN and AVCC and a 0.1 µF capacitor
between VCC and AGND is sufficient to filter any highfrequency line transients on VIN caused by the
MCP1612 switching.
The MCP1612 is packaged in both an 8-pin MSOP and
a thermally-enhanced, 8-pin DFN. The junction
temperature rise above ambient of the MCP1612 is
determined by multiplying the internal power
dissipation by the thermal resistance of the package as
shown:
T RISE = P INT × θ JA
Where:
PINT = MCP1612 internal power dissipation
θJA
= package thermal resistance
It is important to note that the package thermal
resistance is specified for an assumed PCB layout
consisting of four layers with 1 oz. copper on the
internal layers, 2 oz. copper on the external layers and
exposed vias connecting the layers. The thermal
resistance will be higher for a PCB that is constructed
with less layers, less copper weight or fewer vias.
It can be assumed that the difference between the input
power (PIN) and the output power (POUT) is the power
lost in the MCP1612 (PINT) and in the buck inductor,
PIND. Neglecting the small amount of core loss, the
power lost in the buck inductor is defined by the
following equation:
2
P IND = I I ( PEAK ) × RIND
Where:
RIND = inductor winding resistance
COMPENSATION COMPONENTS
The MCP1612 is a peak current mode buck controller
and, therefore, it does not exhibit the second-order
effects associated with the L-C output filter as with a
voltage mode controller. Since a transconductance
error amplifier is used, a simple resistor and capacitor
connected from the output of the amplifier to ground is
all that is needed to provide a stable, high-bandwidth
control loop.
Table 1 shows values for RC and CC for standard circuit
parameters. The values provide a stable control loop
over the entire MCP1612 input voltage, output voltage
and output current range.
TABLE 1:
GENERAL CIRCUIT
PARAMETERS
L
COUT
RC
3.3 µH
10.0 µF
22 kΩ
1000 pF
2.2 µH
4.7 µF
12 kΩ
1000 pF
 2005 Microchip Technology Inc.
CC
PORTABLE/LOAD-SHED
APPLICATIONS
The switching frequency, efficiency, package size and
extremely low shutdown current make the MCP1612
perfectly suited for portable applications or load-shed
applications. Couple the 1.4 MHz switching frequency
that allows the use of small inductors, and filter capacitors with the 8-pin MSOP or the space-saving 8-pin
DFN package and a 1A buck converter that fits into the
most space constrained designs is achievable.
When the MCP1612 is disabled by grounding the
shutdown pin (SHDN), the current draw is only 0.01 µA,
typical. If the load cannot be shed, the quiescent
current (IQ (IOUT = 0 mA)) is only 5 mA, typical. Both of
these low-current draw modes, along with the high
efficiency, extend battery life.
DS00968A-page 3
AN968
PRACTICAL DESIGN EXAMPLE
VOUT_RIPPLE < 15 mV
150
40
100
Gain (dB)
VOUT = 1.8V
200
Phase
20
Gain
50
0
0
-50
-20
-100
-40
IOUT = 0 – 1A
The switching frequency (FSW) of the MCP1612 is
1.4 MHz. The worst-case inductor ripple current is
when VIN is at its maximum. The inductor and output
capacitor must be sized for this worst-case condition.
Therefore:
Duty Cycle =
1.8/4.5
= 0.400
-60
100
Phase Margin (deg)
60
A buck converter with the following design parameters
will be designed using the MCP1612. A schematic of
the circuit appears in Figure 1.
VIN = 2.7V – 4.5V
-150
1000
10000
100000
-200
1000000
Frequency (Hz)
FIGURE 3:
System Bode Plot.
Load Transient Response, IOUT = 100 mA to 800 mA
∆T = 0.40 * 1/1.4 MHz = 286 ns
VIN = 3.3V
Select L = 3.3 µH:
∆IL = ((4.5-1.8)/3.3 µH) * 286 ns = 234 mA
IL(PEAK) =
1 + (1/2 *234 mA)
= 1.12A
The 3.3 µH inductor yields an acceptable ripple current
of about 20% at maximum IOUT. The inductor must be
able to withstand a peak current of 1.12A.
Using an output capacitor (COUT) of 10 µF yields an
approximate output ripple voltage of:
∆VC = (234 mA * 286 ns)/10 µF = 6.68 mV
VESRRIPPLE =
∆VOUT =
234 mA * 10 mΩ
= 2.34 mV
6.68 mV + 2.34 mV
= 9.02 mV
Since the output voltage is compared to the 0.8V
internal reference voltage, a resistor divider needs to
be designed. Selecting RBOT = 200 kΩ, the top resistor
(RTOP) is:
RTOP = 200 kΩ * (1.8/0.8 – 1) = 250 kΩ
VOUT
500 mV/div
VIN
2V/div
100 µs/div
FIGURE 4:
Load Transient Response.
Line Step Response, VIN = 3V to 4.5V
VOUT
100 mV/div
The input capacitor (CIN) is selected to be 10 µF. This
provides adequate filtering on the input.
Figure 2 through Figure 5 show lab performance of the
buck converter built with the components selected
above and compensation values for RC and CC of
22 kΩ and 1000 pF, respectively. Unless otherwise
noted, VIN = 3.3V, VOUT = 1.8V and IOUT = 500 mA.
100
IOUT = 500 mA
100 µs/div
FIGURE 5:
Line Step Response.
CONCLUSION
90
Efficiency (%)
VIN
2V/div
Designing with the MCP1612 is very simple and
straightforward. The buck inductor, output filter
capacitor and resistor divider network are the only
components that need to be calculated.
80
VIN = 3.2V
70
60
VIN = 4.5V
50
40
VIN = 3.8V
30
0.01
0.1
Output Current (A)
FIGURE 2:
DS00968A-page 4
1
The integrated switching and synchronous MOSFETs,
as well as the 1.4 MHz switching speed, make the
MCP1612 buck controller an ideal solution for a spacecontained design. The output voltage is set by a simple
resistor divider and a series resistor capacitor is all that
is required for a high-bandwidth control loop.
Converter Efficiency.
 2005 Microchip Technology Inc.
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DS00968A-page 5
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DS00968A-page 6
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