MAX8744ETJ+ Maxim Integrated Products, MAX8744ETJ+ Datasheet - Page 21

• Manufacturer Part Number: MAX8744ETJ+
• Description: IC CNTRLR PWR SUP QUAD 32TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8744ETJ+

• Applications: Controller, Notebook Computers
• Voltage - Input: 6 ~ 26 V
• Number Of Outputs: 4
• Voltage - Output: 3.3V, 5V, 1 ~ 26 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-TQFN Exposed Pad
• Duty Cycle (max): 99 %
• Output Voltage: 3.315 V, 5.015 V, 2 V to 5.5 V
• Mounting Style: SMD/SMT
• Switching Frequency: 200 KHz, 300 KHz, 500 KHz
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Synchronous Pin: No
• Topology: Boost, Flyback, Forward
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Table 4. FSEL Configuration Table
The heart of each current-mode PWM controller is a
multi-input, open-loop comparator that sums two sig-
nals: the output-voltage error signal with respect to the
reference voltage and the slope-compensation ramp
(Figure 3). The MAX8744/MAX8745 use a direct-sum-
ming configuration, approaching ideal cycle-to-cycle
control over the output voltage without a traditional
error amplifier and the phase shift associated with it.
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 ultraportable 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.
The low-noise forced-PWM mode (SKIP = LDO5) dis-
ables the zero-crossing comparator, which controls the
low-side switch on-time. This forces the low-side gate-
drive waveform to be constantly 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
the switching frequency fairly constant. However, forced-
PWM operation comes at a cost: the no-load 5V supply
current remains between 20mA to 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.
The MAX8744/MAX8745 include a light-load operating
mode control input (SKIP) used to enable or disable
the zero-crossing comparator for both switching regu-
lators. When the zero-crossing comparator is enabled,
the regulator forces DL_ low when the current-sense
OUT
LDO5
FSEL
/V
GND
REF
Light-Load Operation Control ( SKIP )
IN
Supply Controllers for Notebook Computers
. The benefit of forced-PWM mode is to keep
High-Efficiency, Quad-Output, Main Power-
Fixed-Frequency, Current-Mode
______________________________________________________________________________________
SWITCHING FREQUENCY (kHz)
Frequency Selection (FSEL)
Forced-PWM Mode
PWM Controller
500
300
200
inputs detect zero inductor current. This keeps the
inductor from discharging the output capacitors and
forces the regulator to skip pulses under light-load con-
ditions to avoid overcharging the output. When the
zero-crossing comparator is disabled, the regulator is
forced to maintain PWM operation under light-load con-
ditions (forced-PWM).
When pulse-skipping mode is enabled, the on-time of the
step-down controller terminates when the output voltage
exceeds the feedback threshold and when the current-
sense voltage exceeds the idle mode current-sense
threshold. Under light-load conditions, the on-time dura-
tion depends solely on the idle mode current-sense
threshold, which is 20% (SKIP = GND) of the full-load
current-limit threshold set by ILIM, or the low-noise cur-
rent-sense threshold, which is 10% (SKIP = REF) 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 overcharging 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.
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 CSH_ to CSL_. Once V
below the 3mV zero-crossing, current-sense threshold,
the comparator forces DL_ low (Figure 3). This mecha-
nism causes the threshold between pulse-skipping
PFM and nonskipping PWM operation to coincide with
the boundary between continuous and discontinuous
inductor-current operation (also known as the “critical
conduction” point). The load-current level at which
PFM/PWM crossover occurs, I
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
I
LOAD SKIP
Automatic Pulse-Skipping Crossover
Idle Mode Current-Sense Threshold
(
)
=
(
V
IN
2
LOAD(SKIP)
−
V f
V
IN OSC
OUT OUT
CSH
)
V
L
_ - V
, is given by:
CSL
_ drops
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