AND8079 Datasheet, PDF (4/20 Pages) ON Semiconductor – A Low Cost DDR Memory Power Supply Using the NCP1571 Synchronous Buck Converter and a LM358 Based Linear Voltage Regulator

AND8079/D
Component Selection
Input Inductor
The input inductor (L_IN) is used to isolate the input power
supply from the switching portion of the buck regulator. L_IN
also limits the inrush current into the bulk input capacitors
and limits the input current slew rate that results from the
transient load. The inductor blocks the ripple current and
transfers the transient current requirement to the bulk input
capacitor bank.
The design equations for L_IN are listed below and for
connivance an inductance of 1.0 mH is chosen. The cut-off
frequency of the second order LC filter provides adequate
attenuation for the 200 kHz switching frequency of the
NCP1571.
LIN
+
DV
(dIńdt)Max
+
5
V
*
10
2.5
A
V
5 ms + 1.25 mH
f _{* 3db} +
2p
1
Ǹ_LIN
CIN
+
2p
+ 216 Hz
where:
L_IN = input inductor
C_IN = bulk input capacitor(s)
dI/dt = 10 A in 5.0 ms
1
Ǹ_{1 mH}
5400 mF
Input Capacitors
The input filter capacitors provide a charge reservoir that
minimizes the supply voltage variations due to the pulsating
current through the MOSFETs. The input capacitors are
chosen primarily to meet the ripple current rating of the
capacitors.
The design equation is listed below.
ICin(RMS) + ^ǸD (1 * D) Iout2
+ ^Ǹ.5 (1 * .5) 102 + 5 A
where:
D = duty cycle = V_OUT/V_IN = 2.5 V/5.0 V = 0.5
I_OUT = maximum output current
The Rubycon 10 V 1800 mf capacitors have a ripple
current rating of 2.55 A. Thus only 2 of the capacitors are
needed to meet the ripple requirements; however, 3
capacitors were chosen to be conservative.
Output Inductance
The main criterion in selecting the output filter inductance
(L_OUT) is to provide a satisfactory response to the load
transients. The inductance affects the output voltage ripple
by limiting the rate at which the current can either increase
or decrease. The design equation used for selecting L_OUT is
listed below. A 2.2 mH inductor was chosen for the design.
LOUT
+
(VIN
*
VOUT)
DI
+ 2.5 mH
tr
+
(5
V
*
2.5 V)
10 A
10 ms
where:
tr = output transient load time
Output Capacitors
The output capacitors are selected to meet the desired
output ripple requirements. The key specifications for the
capacitors are their ESR (Equivalent Series Resistance) and
ESL (Equivalent Series Inductance). In order to obtain a
good transient response, a combination of low value/high
frequency ceramic capacitors and bulk electrolytic
capacitors are placed as close to the load as possible.
The voltage change during the load current transient is:
DVOUT + DIOUT
^ DIOUT
ǒ Ǔ ESL
Dt
)
ESR
)
tr
COUT
ESR
Empirical data indicates that most of the output voltage
change that results from the load current transients is
determined by the capacitor ESR; therefore, the maximum
allowable ESR can be approximated from the following
equation.
ESR
max
^
DVOUT
DIOUT
+
75 mV
10 A
+
7.5
mW
The number of capacitors is calculated by using the
equation listed below.
Number
of
capacitors
+
ESRCAP
ESR _max
+
19
7.5
mW
mW
+
2.5
The ESR of the Rubycon 6.3 V 1800 mF capacitors is
specified at 19 mW; therefore, 3 capacitors are used in the
design.
MOSFET Selection
The output switch MOSFETs are chosen based on the gate
charge/gate-source threshold voltage, gate capacitance, on
resistance, current rating and the thermal capacity of the
package. In this DDR design, the MOSFETs were chosen for
economical reasons and have a current and power rating that
is much better than needed for this design. In addition, the
MOSFETs selected were verified by measuring the thermal
characteristics of the devices on the PCB.
The power dissipation design equation for selecting the
MOSFETs is given below.
P + IMAX2
RDS(ON)
D ) ^IMAX
VDS
2
Tr
FS
) ^IMAX
VDS
2
Tf
FS
where:
T_r = rise time or turn-on time of MOSFET
T_f = fall time or turn-off time of MOSFET
F_S = switching frequency
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