Datasheet

V
OUT
= 0.8V x
R
fbt
+ R
fbb
R
fbb
C
o
8
2.4A x 0.8V x 2.2 µH x 5V
4 x 2.5V x (5V - 2.5V) x 20mV
I
step
x V
FB
x L x V
IN
4 x V
OUT
x (V
IN
- V
OUT
) x '
Vo_tran
C
O
t
C
O
t PF
LMZ10503EXT
SNVS668G –JUNE 2010–REVISED OCTOBER 2013 www.ti.com
(10)
Three miliohms is a typical R
ESR
value for ceramic capacitors.
The following equation provides a good first pass capacitance requirement for a load transient:
(11)
Where I
step
is the peak to peak load step (for this example I
step
= 10% to 90% of the maximum load), V
FB
= 0.8V,
and ΔV
o_tran
is the maximum output voltage deviation, which is ±20 mV.
Therefore the capacitance requirement for the given design parameters is:
(12)
C
o
≥ 42 µF (13)
In this particular design the output capacitance is determined by the load transient requirements.
Table 1 lists some examples of commercially available capacitors that can be used with the LMZ10503EXT.
Table 1. Recommended Output Filter Capacitors
C
O
(µF) Voltage (V), R
ESR
(mΩ) Make Manufacturer Part Number Case Size
22 6.3, < 5 Ceramic, X5R TDK C3216X5R0J226M 1206
47 6.3, < 5 Ceramic, X5R TDK C3216X5R0J476M 1206
47 6.3, < 5 Ceramic, X5R TDK C3225X5R0J476M 1210
47 10.0, < 5 Ceramic, X5R TDK C3225X5R1A476M 1210
100 6.3, < 5 Ceramic, X5R TDK C3225X5R0J107M 1210
100 6.3, 50 Tantalum AVX TPSD157M006#0050 D, 7.5 x 4.3 x 2.9 mm
100 6.3, 25 Organic Polymer Sanyo 6TPE100MPB2 B2, 3.5 x 2.8 x 1.9 mm
150 6.3, 18 Organic Polymer Sanyo 6TPE150MIC2 C2, 6.0 x 3.2 x 1.8 mm
330 6.3, 18 Organic Polymer Sanyo 6TPE330MIL D3L, 7.3 x 4.3 x 2.8 mm
470 6.3, 23 Niobium Oxide AVX NOME37M006#0023 E, 7.3 x 4.3 x 4.1 mm
Output Voltage Setting
A resistor divider network from V
OUT
to the FB pin determines the desired output voltage as follows:
(14)
R
fbt
is defined based on the voltage loop requirements and R
fbb
is then selected for the desired output voltage.
Resistors are normally selected as 0.5% or 1% tolerance. Higher accuracy resistors such as 0.1% are also
available.
The feedback voltage (at V
OUT
= 2.5V) is accurate to within -2.5% / +2.5% over temperature and over line and
load regulation. Additionally, the LMZ10503EXT contains error nulling circuitry to substantially eliminate the
feedback voltage variation over temperature as well as the long term aging effects of the internal amplifiers. In
addition the zero nulling circuit dramatically reduces the 1/f noise of the bandgap amplifier and reference. The
manifestation of this circuit action is that the duty cycle will have two slightly different but distinct operating points,
each evident every other switching cycle.
Loop Compensation
The LMZ10503EXT preserves flexibility by integrating the control components around the internal error amplifier
while utilizing three small external compensation components from V
OUT
to FB. An integrated type II (two pole,
one zero) voltage-mode compensation network is featured. To ensure stability, an external resistor and small
value capacitor can be added across the upper feedback resistor as a pole-zero pair to complete a type III (three
pole, two zero) compensation network. The compensation components recommended in Table 2 provide type III
compensation at an optimal control loop performance. The typical phase margin is 45° with a bandwidth of 80
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