MAX8627 Maxim Integrated Products, MAX8627 Datasheet - Page 9

• Manufacturer Part Number: MAX8627
• Description: Synchronous Boost Converter
• Manufacturer: Maxim Integrated Products

The MAX8627 is a current-mode step-up converter that
uses a fixed-frequency PWM architecture with True
Shutdown. Consuming only 20µA of quiescent current,
the MAX8627 is highly efficient, with an internal switch
and synchronous rectifier. Shutdown reduces the
quiescent current to less than 1µA. Low quiescent cur-
rent and low noise make this device ideal for powering
portable equipment.
The MAX8627 step-up DC-to-DC switching converter
typically generates a 3V to 5V output voltage from a
1.5V to 4.2V battery input voltage. The IC operates in
bootstrapped mode with the output powering the IC
once the output voltage is equal to, or exceeds, 2.7V.
The default current limit is set at 3.5A to deliver 1A at
5V with an Li+ battery, or 500mA at 5V using a 2-cell
NiCd/NiMH battery. The current limit may be lowered
using an external resistor at ILIM to allow for smaller
components in lower power applications. Internal soft-
start limits the inrush current to less than 500mA under
no-load conditions during startup.
The MAX8627 switches at an internally set frequency of
1MHz allowing for tiny external components. Internal
compensation further reduces the external component
count in cost and space-sensitive applications. The
MAX8627 is optimized for use in DSC and other appli-
cations requiring low quiescent current for maximum
battery life. Figure 1 shows the typical applications
circuit. Figure 2 gives the functional diagram.
The MAX8627 uses a current-mode PWM control
scheme. The voltage difference between FB and an
internal 1.01V reference generates an error signal that
programs the peak inductor current to regulate the
output voltage. The default peak inductor current limit is
typically 3.5A. Inductor current is sensed across the
internal switch and summed with a slope-compensation
signal. The PWM comparator compares this summed
signal to the error amplifier output. At the beginning of
each clock cycle, the n-channel switch turns on until
the PWM comparator trips. During this time, inductor
current ramps up, storing energy in its magnetic field.
When the n-channel switch turns off, the internal
synchronous p-channel rectifier turns on. The inductor
releases the stored energy as the current ramps down
and provides energy to the output.
Low V
_______________________________________________________________________________________
Detailed Description
Boost Converter with True Shutdown
BATT
DC-DC Converter
, 20µA IQ, 1MHz Synchronous
The device operates in PWM when driving medium to
heavy loads. As the load current decreases and crosses
the low-power idle mode threshold, the PWM comparator
and oscillator are disabled. In this low-power idle mode,
switching occurs only as needed to service the output.
This improves the efficiency for light loads and the IC
consumes only 20µA under no-load conditions. At light
loads, the output ripple has a frequency component that
varies with load current. The threshold for entering the
low-power mode is determined by sensing the voltage
drop across the internal switch and comparing it to an
internally generated reference level. This threshold is
approximately 50mA with a 3.6V input and 5V output.
When switching in low-power mode, the inductor
current terminates at zero for each switching cycle. When
operating in this manner, the inductor current is called
discontinuous. In older DC-DC converters, radiated noise
may be higher when inductor current is discontinuous,
because of ringing at the LX switch. The MAX8627 fea-
tures an internal damping switch to minimize ringing at LX
when inductor current is discontinuous. The damping
switch places an impedance across the inductor and
supplies a path to dissipate the resonant energy in the
inductor and capacitor to damp the ringing at the LX.
The damping switch has little effect on output voltage
ripple but does reduce EMI.
At higher loads, the MAX8627 operates in PWM mode.
Regulation is achieved by modulating the MOSFET
switch pulse to control the amount of power transferred
per cycle. Switching harmonics generated by fixed-
frequency operation are consistent and easily filtered.
This is important in noise-sensitive applications.
The MAX8627 matches the load regulation to the volt-
age droop seen during load transients. This is some-
times called voltage positioning. Benefits include lower
peak-to-peak output-voltage deviation for a given load
step without requiring an increase in filter load capaci-
tance. There is minimal voltage droop when transitioning
from a light load to full load and minimum overshoot
when going from full load to light load.
Load-Transient Response/Voltage
Positioning
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