ISL6535
11
FN9255.3
March 3, 2016
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A stable control loop has a gain crossing with close to a
-20dB/decade slope and a phase margin greater than 45°.
Include worst case component variations when determining
phase margin. The mathematical model presented makes a
number of approximations and is generally not accurate at
frequencies approaching or exceeding half the switching
frequency. When designing compensation networks, select
target crossover frequencies in the range of 10% to 30% of the
switching frequency, f
SW
.
Component Selection Guidelines
Output Capacitor Selection
An output capacitor is required to filter the output and supply the
load transient current. The filtering requirements are a function
of the switching frequency and the ripple current. The load
transient requirements are a function of the slew rate (di/dt) and
the magnitude of the transient load current. These requirements
are generally met with a mix of capacitors and careful layout.
Modern microprocessors produce transient load rates above
1A/ns. High frequency capacitors initially supply the transient
and slow the current load rate seen by the bulk capacitors. The
bulk filter capacitor values are generally determined by the ESR
(Effective Series Resistance) and voltage rating requirements
rather than actual capacitance requirements.
High frequency decoupling capacitors should be placed as close
to the power pins of the load as physically possible. Be careful
not to add inductance in the circuit board wiring that could
cancel the usefulness of these low inductance components.
Consult with the manufacturer of the load on specific decoupling
requirements.
Use only specialized low-ESR capacitors intended for switching
regulator applications for the bulk capacitors. The bulk
capacitor’s ESR will determine the output ripple voltage and the
initial voltage drop after a high slew-rate transient. An aluminum
electrolytic capacitor's ESR value is related to the case size with
lower ESR available in larger case sizes. However, the equivalent
series inductance (ESL) of these capacitors increases with case
size and can reduce the usefulness of the capacitor to high
slew-rate transient loading. Unfortunately, ESL is not a specified
parameter. Work with your capacitor supplier and measure the
capacitor’s impedance with frequency to select a suitable
component. In most cases, multiple electrolytic capacitors of
small case size perform better than a single large case capacitor.
Output Inductor Selection
The output inductor is selected to meet the output voltage ripple
requirements and minimize the converter’s response time to the
load transient. The inductor value determines the converter’s
ripple current and the ripple voltage is a function of the ripple
current. The ripple voltage and current are approximated by
Equation 15
:
Increasing the value of inductance reduces the ripple current and
voltage. However, the large inductance values reduce the
converter’s response time to a load transient.
One of the parameters limiting the converter’s response to a load
transient is the time required to change the inductor current. Given
a sufficiently fast control loop design, the ISL6535 will provide
either 0% or 100% duty cycle in response to a load transient. The
response time is the time required to slew the inductor current
from an initial current value to the transient current level. During
this interval the difference between the inductor current and the
transient current level must be supplied by the output capacitor.
Minimizing the response time can minimize the output
capacitance required.
The response time to a transient load is different for the
application of load and the removal of load. The following
equations give the approximate response time interval for
application and removal of a transient load:
Where I
TRAN
is the transient load current step, t
RISE
is the
response time to the application of load, and t
FALL
is the
response time to the removal of load. With a +5V input source,
the worst case response time can be either at the application or
removal of load and dependent upon the output voltage setting.
Be sure to check both of these equations at the minimum and
maximum output levels for the worst case response time.
Input Capacitor Selection
Use a mix of input bypass capacitors to control the voltage
overshoot across the MOSFETs. Use small ceramic capacitors for
high frequency decoupling and bulk capacitors to supply the
current needed each time Q
1
turns on. Place the small ceramic
capacitors physically close to the MOSFETs and between the
drain of Q
1
and the source of Q
2
.
The important parameters for the bulk input capacitor are the
voltage rating and the RMS current rating. For reliable operation,
select a bulk capacitor with voltage and current ratings above the
maximum input voltage and largest RMS current required by the
circuit. The capacitor voltage rating should be at least 1.25x
greater than the maximum input voltage, a voltage rating of 1.5x
greater is a conservative guideline. The RMS current rating
requirement for the input capacitor of a buck regulator is
approximately 1/2 the DC load current.
FIGURE 11. ASYMPTOTIC BODE PLOT OF CONVERTER GAIN
0
F
P1
F
Z2
OPEN LOOP E/A GAIN
F
Z1
F
P2
F
LC
F
CE
COMPENSATION GAIN
GAIN
FREQUENCY
MODULATOR GAIN
CLOSED LOOP GAIN
20
D
MAX
V
IN
V
OSC
----------------------------------log
20
R
2
R
1
--------
log
LOG
LOG
F
0
G
MOD
G
FB
G
CL
V
OUT
= I x ESR
I =
V
IN
- V
OUT
Fs x L
--------------------------------
V
OUT
V
IN
----------------
(EQ. 15)
t
FALL
L
O
I
TRAN
V
OUT
-------------------------------
=t
RISE
L
O
I
TRAN
V
IN
V
OUT
–
--------------------------------
=
(EQ. 16)
