High-Efficiency, Seamless Transition,

Step-Up/Down DC-DC Converter

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11

or oscillation between the boost and buck modes as

seen in other step-up/down converters. See the switch-

ing waveforms for each of the three modes and transi-

tion waveforms in the

Typical Operating Characteristics

section.

Forced-PWM Mode

Drive SKIP high to operate the MAX8625A in forced-

PWM mode. In this mode, the IC operates at a constant

1MHz switching frequency with no pulse skipping. This

scheme is desirable in noise-sensitive applications

because the output ripple is minimized and has a pre-

dictable noise spectrum. Forced PWM consumes higher

supply current at light loads due to constant switching.

Skip Mode

Drive SKIP low to operate the MAX8625A in skip mode

to improve light-load efficiency. In skip mode, the IC

switches only as necessary to maintain the output at

light loads, but still operates with fixed-frequency PWM

at medium and heavy loads. This maximizes light-load

efficiency and reduces the input quiescent current to

37µA (typ).

Do not dynamically transition between skip and FPWM.

The MAX8625A is not designed for dynamic transitions

between FPWM and skip modes. Spikes of negative

inductor current are possible when making these types

of dynamic transitions. The magnitude of the spike

depends on the load and output capacitance. The

MAX8625A has no protection against these types of

negative current spikes.

Load Regulation and Transient Response

During a load transient, the output voltage instantly

changes due to the ESR of the output capacitors by an

amount equal to their ESR times the change in load

current (

then deviates further based on the speed at which the

loop compensates for the load step. Increasing the out-

put capacitance reduces the output-voltage droop. See

the

Capacitor Selection section. The typical application

circuit limits the output transient droop to less than 3%.

See the

Typical Operating Characteristics section.

Soft-Start

Soft-start prevents input inrush current during startup.

Internal soft-start circuitry ramps the peak inductor cur-

rent with an internal DAC in 8ms. Once the output

reaches regulation, the current limit immediately jumps

to the maximum threshold. This allows full load capabil-

ity as soon as regulation is reached, even if it occurs

before the 8ms soft-start time is complete.

When using the MAX8625A at low input voltages (close

to UVLO and < 3V), it is recommended that the ON pin

should not be tied to the BATT or supply voltage node

directly. The ON pin should be held low for > 1ms after

power to the MAX8625A is applied before it is driven

high for normal operation.

Shutdown

Drive ON low to place the MAX8625A in shutdown

mode and reduce supply current to less than 1µA.

During shutdown, OUT is disconnected from IN, and

LX1 and LX2 are connected to GND. Drive ON high for

normal operation.

Fault and Thermal Shutdown

The MAX8625A contains current-limit and thermal shut-

down circuitry to protect the IC from fault conditions.

When the inductor current exceeds the current limit (2A

for the MAX8625A), the converter immediately enters

phase 3 and an internal 100ms timer starts. The con-

verter continues to commutate through the three phas-

es, spending most of its time in phase 1 and phase 3. If

the overcurrent event continues and the output is out of

regulation for the duration of the 100ms timer, the IC

enters shutdown mode and the output latches off. ON

must then be toggled to clear the fault. If the overload

is removed before the 100ms timer expires, the timer is

cleared and the converter resumes normal operation.

The thermal-shutdown circuitry disables the IC switching

if the die temperature exceeds +165°C. The IC begins

soft-start once the die temperature cools by 15°C.