EL7554IREZT13 INTERSIL [Intersil Corporation], EL7554IREZT13 Datasheet - Page 12

• Manufacturer Part Number: EL7554IREZT13
• Description: Monolithic 4 Amp DC-DC Step-Down Regulator
• Manufacturer: INTERSIL [Intersil Corporation]

output capacitor with the ESR to satisfy the output ripple
∆V
When output has a step load change ∆I
drop is ESR*∆I
loop has the chance to respond. The higher the output
capacitance, the lower the voltage drop is. Also, higher loop
bandwidth will generate less voltage drop. Experiment with
the transient response (see Figure 15) to determine the final
values of output capacitance.
Like the input capacitor, it is recommended to use X5R or
X7R type of ceramic capacitors, or SPCAP or POSCAP type
of Polymer capacitors for the low ESR and high capacitance.
Generally, the AC current rating of the output capacitor is not
a concern because the RMS current is only 1/√12 of ∆I
This is easily satisfied.
LOOP COMPENSATION
Current mode converter forces the inductor current
proportional to the error signal, thus gets rid of the 2nd order
effect formed by the inductor and output capacitor. The PWM
comparator and the inductor form an equivalent
transconductance amplifier. So, a simple Type 1
compensator is good enough to generate a high bandwidth
stable converter. The compensation capacitor and resister
are decided by:
where:
• GM
• V
• I
• C
• GM
• F
∆V
C
R
R
C
C
OUT
GM
GM
best performance, set this value to about one-tenth of the
switching frequency.
O
OUT
O
C
OUT
OUT
=
=
requirement:
=
is the intended crossover frequency of the loop. For
PWM
PWM
EA
EA
V
---------------------------------------------------------------- -
2
=
∆I
output current
FB
×
output voltage
is output capacitance
V
--------------- -
is the transconductance of the error amplifier,
= 120µs
R
I
L
OUT
OUT
×
×
π
OUT
is the transconductance of the PWM comparator,
= 120s
GM
ESR
×
F
O
C
×
PWM
. Then V
C
--------------- -
×
C
OUT
I
OUT
C
×
GM
O
EA
will drop even further before the
12
O
, the initial voltage
L
.
EL7554
Design Example
A 5V to 1.8V converter at 4A is needed.
The input capacitor or combination of capacitors has to be
able to take about 1/2 of the output current, e.g., 2A. TDK’s
C3216X5RIA106M is rated at 2.7A, 6.3V, meeting the above
criteria using 2 generators less input voltage ripple.
∆I
L = 2.2µH. TDK RLF7030-2R2M5R4 has the required
current rating.
L = 2.2µH yields about 0.9A inductor ripple current. 47µF
ceramic capacitor has less than 5mΩ of ESR easily
satisfying by the requirement. ESR is not the only factor
deciding the output capacitance. As discussed earlier, output
voltage droops less with more capacitance when converter is
in load transient. Multiple iterations may be needed before
final components are chosen.
50kHz is the intended crossover frequency. With the
conditions R
R
For convenience, Table 3 lists the compensation values for
frequently used output voltages.
L
1. Choose the input capacitor
2. Choose the inductor. Set the converter switching
3. Choose the output capacitor
4. Loop compensation
C
L
=
= 1A yields 1.72µH. Leave some margin and choose
= 2.32kΩ and C
frequency at 600kHz:
(
------------------------------------------- -
V
V
IN
V
IN
O
3.3
2.5
1.8
1.5
1.2
0.8
1
–
×
(V)
V
∆I
C
O
TABLE 3. COMPENSATION VALUES
L
)
and C
×
×
F
V
S
O
C
C
= 0.018pF
are calculated as:
R
C
4.22
3.24
2.32
1.91
1.54
1.27
1.02
(kΩ)
C
0.018
0.018
0.018
0.018
0.018
0.018
0.018
January 31, 2005
C
(µF)
FN7360.2