FAN6520A_06 Datasheet, PDF (9/15 Pages) Fairchild Semiconductor – Single Synchronous Buck PWM Controller

100
F_Z1 F_Z2 F_P1 F_P2
80
OPEN LOOP
60
ERROR AMP GAIN
40
20LOG
20 ^(R2/R1)
0
MODULATOR
GAIN
-20
20LOG
(V_IN/DV_OSC)
COMPENSATION
GAIN
CLOSED LOOP
-40
GAIN
-60
F_LC
F_ESR
10
100
1K
10K 100K 1M
10M
FREQUENCY (Hz)
Figure 8. Asymptotic Bode Plot of Converter Gain
An output capacitor is required to filter the output and
supply the load transient current. The filtering require-
ments 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.
Component Selection
Output Capacitors (C_OUT)
Modern components and loads are capable of producing
transient load rates above 1A/ns. High-frequency capac-
itors initially supply the transient and slow the current
load rate seen by the bulk capacitors. Effective Series
Resistance (ESR) and voltage rating are typically the
prime considerations for the bulk filter capacitors, 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 performance of these low-inductance
components. Consult with the load manufacturer on spe-
cific decoupling requirements. Use only specialized low-
ESR capacitors intended for switching-regulator applica-
tions for the bulk capacitors. The bulk capacitor’s ESR
determines the output ripple voltage and the initial volt-
age 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; how-
ever, 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. Since ESL is not a specified parameter, work
with the capacitor supplier and measure the capacitor’s
impedance with frequency to select a suitable compo-
nent. Generally, multiple small-case electrolytic capaci-
tors perform better than a single large-case capacitor.
Output Inductor (L_OUT)
The output inductor is selected to meet the output volt-
age 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 volt-
age (ΔV) and current (ΔI) are approximated by the fol-
lowing equations:
ΔI
=
⎛
⎜
⎝
V-----_IF-N---S--^–_{-W----V--×--O---L-U----T-⎠⎟⎞}
× ^V-----O----^U---^T--
V_IN
ΔV ≈ ESR × ΔI
(9)
Increasing the inductance value reduces the ripple cur-
rent and voltage, but also reduces the converter’s ability
to quickly respond to a load transient. One of the param-
eters 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
FAN6520A provides 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.
Depending on whether there is a load application or a
load removal, the response time to a load transient
(I_STEP) is different. The following equations give the
approximate response time interval for application and
removal of a transient load:
T_RISE
=
---L-----^×-----I--^S---^T---^E----^P----
V_IN – V_OUT
(10)
T_FALL
=
L-----×_{-----I--S----T---E---P--}
V_OUT
(11)
where T_RISE is the response time to the application of a
positive I_STEP and T_FALL is the response time to a load
removal (negative I_STEP). The worst-case response time
can be either at application or removal of load. 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 volt-
age overshoot across the MOSFETs. Use small ceramic
capacitors for high-frequency decoupling and bulk
capacitors to supply the current needed each time Q1
turns on. Place the small ceramic capacitors physically
close to the MOSFETs and between the drain of Q1 and
the source of Q2. The important parameters for the bulk
input capacitor are the voltage rating and the RMS cur-
rent rating. For reliable operation, select the bulk capaci-
tor with voltage and current ratings above the maximum
input voltage and the largest RMS current required by
the circuit. The capacitor voltage rating should be at least
©2005 Fairchild Semiconductor Corporation
FAN6520A Rev. 1.0.5
9
www.fairchildsemi.com