MAX1530_09 Datasheet, PDF (24/33 Pages) Maxim Integrated Products – Multiple-Output Power-Supply Controllers for LCD Monitors

Multiple-Output Power-Supply
Controllers for LCD Monitors
VL and AGND at ILIM. The threshold is approximately
1/5th the voltage on ILIM over a range of 0.25V to 3V:
I_VALLEY × R_{DS(ON)_HOT} < 0.2 × V_ILIM × (1− K)
K is the accuracy of the current-limit threshold, which is
20% when the threshold is 250mV.
For example, Figure 1’s N1 MOSFET has a maximum
R_DS(ON) at room temperature of 145mΩ and an esti-
mate of its maximum at our chosen maximum tempera-
ture of +85°C is 188mΩ. Since the inductor ripple
current is 0.5A, the valley current through the MOSFET
is 1.25A. So the maximum valley current-sense signal is
235mV, which is too high to work with the 190mV mini-
mum of the default current-limit threshold. Adding a
divider at ILIM (R12 and R13) adjusts the ILIM voltage to
1.7V and the current-limit threshold to 340mV, providing
more than adequate margin for threshold accuracy.
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduce noise and voltage
ripple on the input caused by the regulator’s switching.
It is usually selected according to input ripple current
requirements and voltage rating, rather than capaci-
tance value. The input voltage and load current deter-
mine the RMS input ripple current (I_RMS):
I_RMS = I_LOAD ×
V_OUT × (V_IN − V_OUT)
V_IN
The worst case is I_RMS = 0.5 × I_LOAD, which occurs at
V_IN = 2 × V_OUT.
For most applications, ceramic capacitors are used
because of their high ripple current and surge current
capabilities. For long-term reliability, choose an input
capacitor that exhibits less than +10°C temperature
rise at the RMS input current corresponding to the max-
imum load current.
Output Capacitor
The output capacitor and its equivalent series resis-
tance (ESR) affect the regulator’s loop stability, output
ripple voltage, and transient response. The
Compensation Design section discusses the output
capacitance requirement based on the loop stability.
This section deals with how to determine the output
capacitance and ESR needs according to the ripple
voltage and load transient requirements.
The output voltage ripple has two components: varia-
tions in the charge stored in the output capacitor, and
the voltage drop across the capacitor’s ESR caused by
the current into and out of the capacitor:
V_RIPPLE = V_RIPPLE(ESR) + V_RIPPLE(C)
V_RIPPLE(ESR) = I_RIPPLE × R_ESR
V_RIPPLE(C)
=
8×
I_RIPPLE
C_OUT ×
f_SW
where C_OUT is the output capacitance, and R_ESR is the
ESR of the output capacitor. In Figure 1’s circuit, the
inductor ripple current is 0.5A. Assume the voltage-rip-
ple requirement is 2% (peak-to-peak) of the 3.3V out-
put, which corresponds to 66mV total peak-to-peak
ripple. Assuming that the ESR ripple component and
the capacitive ripple component each should be less
than 50% of the 66mV total peak-to-peak ripple, then
the ESR should be less than 66mΩ and the output
capacitance should be more than 7.6µF to meet the
total ripple requirement. A 22µF ceramic capacitor with
ESR (including PC board trace resistance) of 10mΩ is
selected for the standard application circuit in Figure 1,
which easily meets the voltage ripple requirement.
The step-down regulator’s output capacitance and ESR
also affect the voltage undershoot and overshoot when
the load steps up and down abruptly. The undershoot
and overshoot have three components: the voltage
steps caused by ESR, the voltage undershoot and
overshoot due to the current-mode control’s AC load
regulation, and the voltage sag and soar due to the
finite capacitance and inductor slew rate.
The amplitude of the ESR steps is a function of the load
step and the ESR of the output capacitor:
V_{ESR_ STEP} = ΔI_LOAD × R_ESR
The amplitude of the sag due to the finite output capac-
itance and inductor slew rate is a function of the load
step, the output capacitor value, the inductor value, the
input-to-output voltage differential, and the maximum
duty cycle:
V_{SAG _ LC}
=
2
×
C_OUT
L × (ΔI_LOAD)²
× (V_IN(MIN) × D_MAX
-
V_OUT )
The amplitude of the undershoot due to the AC load
regulation is a function of the high-side MOSFET
R_DS(ON), the gain of the current-sense amplifier A_VCS,
the change of the slope compensation during the under-
shoot (ΔSC_UNDER), the transconductance of the error
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