MAX8745 datasheet(21/36 Pages) MAXIM

21 / 36 page

High-Efficiency, Quad Output, Main Power-

Supply Controllers for Notebook Computers

______________________________________________________________________________________

Fixed-Frequency, Current-Mode

PWM Controller

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.

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 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.

Forced-PWM Mode

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 VOUT/VIN. The benefit of forced-PWM mode is to keep

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.

Light-Load Operation Control (

SKIP)

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

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).

Idle Mode Current-Sense Threshold

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.

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 CSH_ to CSL_. Once VCSH_ - VCSL_ drops

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, ILOAD(SKIP), is given by:

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

LOAD SKIP

OUT

IN OSC

()

−

FSEL

SWITCHING FREQUENCY (kHz)

LDO5

500

REF

300

GND

200

Table 4. FSEL Configuration Table

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