MAX17083 Просмотр технического описания (PDF) - Maxim Integrated

Номер в каталоге

Компоненты Описание

производитель

MAX17083

Low-Voltage, Internal Switch, Step-Down Regulator

Maxim Integrated 

Low-Voltage, Internal Switch,

Step-Down Regulator

SMPS Detailed Description

Fixed-Frequency,

Current-Mode PWM Controller

The heart of the current-mode PWM controller is a multi-

stage, open-loop comparator that compares the output

voltage-error signal with respect to the reference volt-

age, the current-sense signal, and the slope compensa-

tion ramp (Figure 2). The MAX17083 uses a direct-

summing 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 (FREQ)

The FREQ input selects the PWM mode switching fre-

quency. FREQ is a four-level input to set the regulator

switching frequency. The regulator’s switching frequen-

cy is set according to Table 1, and latched at the

beginning of soft-start. High-frequency (FREQ = VCC)

operation optimizes the application for the smallest

component size, trading off efficiency due to higher

switching losses. This might be acceptable in ultra-

portable devices where the load currents are lower.

Low-frequency (FREQ = GND) operation offers the best

overall efficiency at the expense of component size and

board space.

Table 1. MAX17083 FREQ Table

FREQ PIN SWITCHING

SELECT FREQ, fSW

SOFT-START

TIME (ms)

1833/fSW

VCC

1.5MHz

1.22

Open

1MHz

1.83

REF

750 kH z

2.44

GND

500 kH z

3.67

STARTUP

BLANKING

TIME (ms)

3055/fSW

2.0

3.1

4.1

6.1

FB Regulation Selection (SET)

The SET input selects one of the four preset feedback

regulation voltage levels. The SET pin is a four-level

input signal to set the FB regulation voltage. The regu-

lator’s feedback regulation voltage is set according to

Table 2, and latched at the beginning of soft-start.

Table 2. MAX17083 SET Table

SET PIN SELECT

VCC

Open

REF

GND

FB REGULATION VOLTAGE (V)

1.8

1.5

1.1

0.75

Adjustable Output-Voltage Operation Mode

The MAX17083 produces an adjustable 0.75V to 2.7V

output voltage from the system’s 3.3V or 5V input sup-

ply by using a resistive feedback divider. Set FB to

0.75V (SET = GND) in adjustable mode.

Light-Load Operation

An inherent automatic switchover to pulse-skipping

(PFM operation) takes place at light loads. 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 current during the off-time. Once the cur-

rent through the low-side MOSFET drops below 100mA,

the zero-crossing comparator, turns off the low-side

MOSFET. This prevents the inductor from discharging

the output capacitors and forces the switching regula-

tor to skip pulses under light-load conditions to avoid

overcharging the output.

Idle-Mode Current-Sense Threshold

When MAX17083 operates in pulse-skipping mode, the

on-time of the step-down controller terminates when

both the output voltage exceeds the feedback thresh-

old, and 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. This forces the con-

troller to source a minimum amount of power with each

cycle. To avoid overcharging the output, another on-

time cannot be initiated until the output voltage drops

below the feedback threshold. Since the zero-crossing

comparator prevents the switching regulator from sink-

ing current, the MAX17083 switching regulator must

skip pulses. Therefore, the controller regulates the

valley of the output ripple under light-load conditions.

The minimum idle-mode current requirement causes

the threshold between pulse-skipping PFM operation

and constant 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 equivalent

to half the idle-mode current threshold (see the

Electrical Characteristics table for the idle-mode thresh-

old of the regulator). The switching waveforms can

appear noisy and asynchronous at light-load pulse-

skipping operation, but this is a normal operating con-

dition that results in high light-load efficiency.

Trade-offs in PFM noise and light-load efficiency are

made by varying the inductor value. Generally, low

inductor values produce a broader efficiency vs. load

10 ______________________________________________________________________________________

Share Link: