MAX8627 Maxim Integrated Products, MAX8627 Datasheet - Page 10

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

Low V
Boost Converter with True Shutdown
The term “positioning” refers to setting the output volt-
age to a level that is dependent on load current (see
Figure 3). At minimum load, the output voltage is set to
a slightly higher than nominal level. At full load, the out-
put voltage is slightly lower than the nominal level. With
voltage positioning, the total voltage deviation during a
transient is significantly improved over traditional high-
gain control loops. Traditional high-gain loops use inte-
grators that maximize gain at low frequencies to
provide tight DC-load regulation; however, due to the
capacitive element in the feedback loop, these high-
gain amplifiers typically take hundreds of micro-
seconds to respond to a load step and return to steady
state. As a result, the voltage can droop by as much as
6% or more during the recovery time. In portable equip-
ment where the output load can change frequently, and
the amount of output capacitance that can fit is limited,
this can result in a wide short-term output fluctuation
(see Figure 4).
Voltage positioning on the MAX8627 allows up to 3%
(typ) of load regulation and no further transient droop
(Figures 3 and 4). Thus, during load transients the volt-
age delivered remains within specification more effec-
tively than other regulators that might have tighter DC
accuracy. In systems with high-speed CPUs, thou-
sands of system clock cycles can occur during the time
it takes a traditional high-gain loop to respond to a load
step. Consequently, 3% load regulation with no tran-
sient droop is better suited to such systems than a
power supply that may spec 1% DC load regulation,
but then exhibits 6% or more of transient droop during
load steps (see the Load Transient Response in the
Typical Operating Characteristics section).
Figure 3. Load-Regulation Specification
10
______________________________________________________________________________________
5.02
5.00
4.98
4.96
4.94
4.92
4.90
4.88
4.86
4.84
4.82
4.80
0
BATT
V
500
IN
vs. LOAD CURRENT
OUTPUT VOLTAGE
= 4V
LOAD CURRENT (mA)
1000
, 20µA IQ, 1MHz Synchronous
V
IN
= 1.8V
V
IN
= 2.8V
1500
2000
Connecting ON to GND or logic low places the
MAX8627 in shutdown mode and reduces supply cur-
rent to 0.1µA. In shutdown, the control circuitry, internal
switching MOSFET, and synchronous rectifier turn off
and LX becomes high impedance. Connect ON to
BATT or logic high for normal operation.
The MAX8627 has an internal synchronous rectifier,
which allows for conversion efficiencies as high as
95%. In conventional boost circuits, the body diode of
the synchronous rectifier is forward biased in shutdown
and allows current flow from the battery to the output. If
the load cannot be shut down, an external switch is
required to avoid depleting the battery during shut-
down. A proprietary design in the MAX8627 allows the
synchronous rectifier to provide True Shutdown with no
additional components. This allows the output to fall to
GND in shutdown and removes any connection
between the input and output.
The MAX8627 has internal soft-start circuitry that elimi-
nates inrush current at startup, reducing transients on
the input source. Soft-start is particularly useful for
higher impedance input sources, such as Li+ and alka-
line cells. The soft-start duration is proportional to the
size of the output capacitor and load resistance with a
typical time of 5.25ms. See the Typical Operating
Characteristics section for plots of Soft-Start Time vs.
Load Current and Soft-Start Time vs. Input Voltage.
Inrush current is controlled during startup and initially
set to 500mA. After 1000 clock cycles, if the output volt-
age is not within regulation, the startup current limit is
Figure 4. Transient-Response Comparison
9%
3%
(a) HIGH-GAIN DC LOAD REGULATION WITH POOR TRANSIENT RESPONSE
(b) VOLTAGE POSITIONING WITH DC LOAD REGULATION
True Shutdown
Soft-Start