MAX1533A Datasheet, PDF (22/38 Pages) Maxim Integrated Products – High-Efficiency, 5x Output, Main Power-Supply Controllers for Notebook Computers

High-Efficiency, 5x Output, Main Power-Supply
Controllers for Notebook Computers
Frequency Selection (FSEL)
The FSEL input selects the PWM-mode switching fre-
quency. Table 4 shows the switching frequency based
on FSEL connection. High-frequency (500kHz) operation
optimizes the application for the smallest component
size, trading off efficiency due to higher switching losses.
This may be acceptable in ultra-portable devices where
the load currents are lower. Low-frequency (200kHz)
operation offers the best overall efficiency at the expense
of component size and board space.
Forced-PWM Mode
The low-noise forced-PWM mode disables the zero-
crossing comparator, which controls the low-side switch
on-time. This forces the low-side gate-drive waveform to
constantly be the complement of the high-side gate-
drive waveform, so the inductor current reverses at light
loads while DH_ maintains a duty factor of V_OUT / V_IN.
The benefit of forced-PWM mode is to keep the switch-
ing frequency fairly constant. However, forced-PWM
operation comes at a cost: the no-load 5V supply current
remains between 15mA and 50mA, depending on the
external MOSFETs and switching frequency.
Forced-PWM mode is most useful for avoiding audio-
frequency noise and improving load-transient
response. Since forced-PWM operation disables the
zero-crossing comparator, the inductor current revers-
es under light loads.
Light-Load Operation Control (SKIP)
The MAX1533A/MAX1537A include a light-load operat-
ing-mode control input (SKIP) used to independently
enable or disable the zero-crossing comparator for
both controllers. When the zero-crossing comparator is
enabled, the controller forces DL_ low when the cur-
rent-sense inputs detect zero inductor current. This
keeps the inductor from discharging the output capaci-
tors and forces the controller to skip pulses under light-
load conditions to avoid overcharging the output. When
the zero-crossing comparator is disabled, the controller
is forced to maintain PWM operation under light-load
conditions (forced-PWM).
Table 4. FSEL Configuration Table
FSEL
V_CC
REF
GND
SWITCHING FREQUENCY
500kHz
300kHz
200kHz
Idle-Mode Current-Sense Threshold
The on-time of the step-down controller terminates
when the output voltage exceeds the feedback thresh-
old and when the current-sense voltage exceeds the
idle-mode current-sense threshold. Under light-load
conditions, the on-time duration depends solely on the
idle-mode current-sense threshold, which is approxi-
mately 20% of the full-load current-limit threshold set by
ILIM_. This forces the controller to source a minimum
amount of power with each cycle. To avoid overcharg-
ing the output, another on-time cannot begin until the
output voltage drops below the feedback threshold.
Since the zero-crossing comparator prevents the
switching regulator from sinking current, the controller
must skip pulses. Therefore, the controller regulates the
valley of the output ripple under light-load conditions.
Automatic Pulse-Skipping Crossover
In skip mode, an inherent automatic switchover to PFM
takes place at light loads (Figure 4). This switchover is
affected by a comparator that truncates the low-side
switch on-time at the inductor current’s zero crossing.
The zero-crossing comparator senses the inductor cur-
rent across the low-side MOSFET (PGND to LX_). Once
V_PGND - V_LX_ drops below the 3mV zero-crossing cur-
rent-sense threshold, the comparator forces DL_ low
(Figure 3). This mechanism causes the threshold
between pulse-skipping PFM and nonskipping PWM
operation to coincide with the boundary between con-
tinuous and discontinuous inductor-current operation
(also known as the “critical conduction” point). The
load-current level at which PFM/PWM crossover
occurs, I_LOAD(SKIP), is given by:
I_LOAD(SKIP)
=
V_OUT (V_IN − V_OUT)
2 × V_IN × f_SW × L
The switching waveforms may appear noisy and asyn-
chronous when light loading causes pulse-skipping
operation, but this is a normal operating condition that
results in high light-load efficiency. Trade-offs in PFM
noise vs. light-load efficiency are made by varying the
inductor value. Generally, low inductor values produce
a broader efficiency vs. load curve, while higher values
result in higher full-load efficiency (assuming that the
coil resistance remains fixed) and less output voltage
ripple. Penalties for using higher inductor values
include larger physical size and degraded load-tran-
sient response (especially at low input-voltage levels).
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