Datasheet

MAX8627
Low V
BATT
, 20µA IQ, 1MHz Synchronous
Boost Converter with True Shutdown
11
Maxim Integrated
incremented by 230mA. If after 13 increments, the out-
put is still not in regulation, the MAX8627 latches off,
assuming a short-circuit overload condition exists on
the output. To clear the latched condition, cycle ON.
Fault Protection
The MAX8627 has a fault-overload protection. After
soft-start, the device is set to detect an out-of-regula-
tion state that could be caused by an overload. If the
output remains faulted for 65ms, then the MAX8627
latches off. Fault-detection circuitry is disabled during
soft-start. If short on the output exists before the
MAX8627 is turned ON, the converter completes the
soft-start sequence and latches off. The converter can
be reinitialized from a fault latch-off state by toggling
the ON pin or by cycling the input power.
BATT/Damping Switch
The MAX8627 features an internal damping switch to
minimize ringing at LX caused by the resonant circuit
formed by the inductor and output capacitor in discon-
tinuous conduction mode. This occurs at light loads.
The damping switch connects across the inductor
when the inductor energy is depleted and supplies a
path to dissipate the resonant energy. Damping LX
ringing does not change the output ripple but reduces
EMI.
Applications Information
Setting the Output Voltage
To set the output voltage to between 3V and 5V, con-
nect FB to the center of an external resistor voltage-
divider between OUTS and GND, as shown in Figure 1.
Select the value of R2 less than 500kΩ, and then calcu-
late the value for R1 as follows:
where V
FB
is the FB regulation voltage, 1.015V (typ).
Inductor Selection
In most step-up converter designs, a reasonable inductor
value can be derived from the following equation,
which sets peak-to-peak inductor current at 1/2 the DC
inductor current:
where f
SW
is the switching frequency (1MHz), and D is
the duty factor given by D = 1 - ( V
BATT
/ V
OUT
).
Using L from the equation above results in a peak-to-
peak inductor current ripple of 0.5 x I
OUT
/ (1 - D), and
a peak inductor current of 1.25 x I
OUT
/ (1 - D). Ensure
the peak (saturation) current rating of the inductor
meets or exceeds this requirement.
The recommended inductance range for the MAX8627 is
1µH to 4.7µH. See Table 1 for recommended inductors.
Capacitor Selection
Output Capacitor
Output capacitors C3 and C4 in Figure 1 are required to
keep the output voltage ripple small and to ensure regu-
lation loop stability. The output capacitors must have low
impedance at the switching frequency. Ceramic capaci-
tors are highly recommended due to their small size and
low ESR. Make sure the output capacitors maintain their
capacitance over DC bias and the desired operating
temperature range. Ceramic capacitors with X5R or X7R
temperature characteristics generally perform well. Two
22µF ceramic capacitors in parallel are recommended.
Alternatively, two 47µF tantalum capacitors with 70mΩ or
lower ESR may be used.
Input Capacitor
Input capacitors C1 and C2 reduce the current peaks
drawn from the battery or input power source and reduce
switching noise in the IC. The impedance of the input
capacitors at the switching frequency should be kept
very low. Ceramic capacitors are highly recommended
due to their small size and low ESR. Make sure the input
capacitors maintain their capacitance over DC bias and
the desired operating temperature range. Ceramic
capacitors with X5R or X7R temperature characteristics
generally perform well. Two 22µF ceramic capacitors are
recommended.
L
xV xDx D
Ixf
BATT
OUT MAX SW
=
−21()
()
RRx
V
V
OUT
FB
12 1=
⎛
⎝
⎜
⎞
⎠
⎟
−
Table 1. Recommended Inductors
PART INDUCTANCE (µH) RATED CURRENT (mA)
SIZE:
L (mm, typ) x W (mm, typ) x H (mm, max)
TOKO A918CY 1.0 3500 6.3 x 6.2 x 2
TOKO A997AS 1.5 2150 3.8 x 3.8 x 1.8