Rainbow Electronics MAX608 - Manual (Page 10)

MAX608
Power Transistor Selection
Use an N-channel MOSFET power transistor with the
MAX608.
Use logic-level or low-threshold N-FETs to ensure the
external N-channel MOSFET (N-FET) is turned on com-
pletely and that start-up occurs. N-FETs provide the
highest efficiency because they do not draw any DC
gate-drive current.
When selecting an N-FET, some important parameters
to consider are the total gate charge (Q
g ), on-resis-
tance (r DS(ON) ), reverse transfer capacitance (C RSS ),
maximum drain to source voltage (V DS max), maximum
gate to source voltage (V GS max), and minimum thresh-
old voltage (V TH min).
Q g takes into account all capacitances associated with
charging the gate. Use the typical Q g value for best
results; the maximum value is usually grossly over-
specified since it is a guaranteed limit and not the mea-
sured value. The typical total gate charge should be
50nC or less. With larger numbers, the EXT pins may
not be able to adequately drive the gate. The EXT
rise/fall time varies with different capacitive loads as
shown in the
Typical Operating Characteristics.
The two most significant losses contributing to the
N-FET’s power dissipation are I
2 R losses and switching
losses. Select a transistor with low r DS(ON) and low
C RSS to minimize these losses.
Determine the maximum required gate-drive current
from the Q
g specification in the N-FET data sheet.
Select an N-FET with a BV DSS > V OUT , BV GSS > V OUT ,
and a minimum V TH of 0.5V below the minimum input
voltage.
When using a power supply that decays with time
(such as a battery), the N-FET transistor will operate in
its linear region when the voltage at EXT approaches
the threshold voltage of the FET, dissipating excessive
power. Prolonged operation in this mode may damage
the FET. To avoid this condition, make sure V
EXT is
above the V TH of the FET, or use a voltage detector
(such as the MAX8211) to put the IC in shutdown mode
once the input supply voltage falls below a predeter-
mined minimum value. Excessive loads with low input
voltages can also cause this condition.
The MAX608’s maximum allowed switching frequency
during normal operation is 300kHz. However, at start-
up, the maximum frequency can be 500kHz, so the
maximum current required to charge the N-FET’s gate
is f(max) x Q
g (typ). Use the typical Q g number from the
transistor data sheet. For example, the MMFT3055EL
has a Q
g (typ) of 7nC (at V GS = 5V), therefore the cur-
rent required to charge the gate is:
I GATE (max) = (500kHz) (7nC) = 3.5mA.
Figure 2a’s application circuit uses a 4-pin MMFT3055EL
surface-mount N-FET that has 150m
Ω on-resistance with
4.5V V GS , and a guaranteed V TH of less than 2V. Figure
2c’s application circuit uses an Si6426DQ logic-level N-
FET with a threshold voltage (V
TH ) of 1V.
Diode Selection
The MAX608’s high switching frequency demands a
high-speed rectifier. Schottky diodes such as the
1N5817–1N5822 are recommended. Make sure the
Schottky diode’s average current rating exceeds the
peak current limit set by R
SENSE , and that its break-
down voltage exceeds V OUT . For high-temperature
applications, Schottky diodes may be inadequate due
to their high leakage currents; high-speed silicon
diodes such as the MUR105 or EC11FS1 can be used
instead. At heavy loads and high temperatures, the
benefits of a Schottky diode’s low forward voltage may
outweigh the disadvantage of high leakage current.
Capacitor Selection
Output Filter Capacitor
The primary criterion for selecting the output filter capac-
itor (C4) is low effective series resistance (ESR). The
product of the peak inductor current and the output filter
capacitor’s ESR determines the amplitude of the ripple
seen on the output voltage. Two OS-CON 100µF, 16V
output filter capacitors in parallel with 35m
Ω of ESR each
typically provide 75mV ripple when stepping up from 2V
to 5V at 500mA (Figure 2a). Smaller-value and/or higher-
ESR capacitors are acceptable for light loads or in appli-
cations that can tolerate higher output ripple.
Since the output filter capacitor’s ESR affects efficien-
cy, use low-ESR capacitors for best performance. See
Table 1 for component selection.
Input Bypass Capacitors
The input bypass capacitor (C1) reduces peak currents
drawn from the voltage source and also reduces noise
caused by the switching action of the MAX608 at the
voltage source. The input voltage source impedance
determines the size of the capacitor required at the
OUT input. As with the output filter capacitor, a low-ESR
capacitor is recommended. For output currents up to
1A, 150µF (C1) is adequate, although smaller bypass
capacitors may also be acceptable.
Bypass the IC with a 0.1µF ceramic capacitor (C2)
placed as close as possible to the OUT and GND pins.
Reference Capacitor
Bypass REF with a 0.1µF capacitor (C3). REF can
source up to 100µA of current for external loads.
5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
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