MAX608 Maxim, MAX608 Datasheet - Page 10

• Manufacturer Part Number: MAX608
• Description: 5V or Adjustable / Low-Voltage / Step-Up DC-DC Controller
• Manufacturer: Maxim

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
tance (r
maximum drain to source voltage (V
gate to source voltage (V
old voltage (V
Q
charging the gate. Use the typical Q
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
losses. Select a transistor with low r
C
Determine the maximum required gate-drive current
from the Q
Select an N-FET with a BV
and a minimum V
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
above the V
(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
transistor data sheet. For example, the MMFT3055EL
has a Q
rent required to charge the gate is:
5V or Adjustable, Low-Voltage,
Step-Up DC-DC Controller
10
RSS
g
takes into account all capacitances associated with
______________________________________________________________________________________
to minimize these losses.
DS(ON)
g
(typ) of 7nC (at V
g
specification in the N-FET data sheet.
TH
g
TH
(typ). Use the typical Q
), reverse transfer capacitance (C
of the FET, or use a voltage detector
min).
TH
Power Transistor Selection
of 0.5V below the minimum input
GS
DSS
GS
max), and minimum thresh-
= 5V), therefore the cur-
> V
2
R losses and switching
OUT
DS
g
, BV
g
number from the
DS(ON)
max), maximum
value for best
GSS
g
), on-resis-
and low
> V
EXT
RSS
OUT
is
),
,
Figure 2a’s application circuit uses a 4-pin MMFT3055EL
surface-mount N-FET that has 150mΩ on-resistance with
4.5V V
2c’s application circuit uses an Si6426DQ logic-level N-
FET with a threshold voltage (V
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
down voltage exceeds V
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.
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 35mΩ 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.
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.
Bypass REF with a 0.1µF capacitor (C3). REF can
source up to 100µA of current for external loads.
GS
I
GATE
, and a guaranteed V
(max) = (500kHz) (7nC) = 3.5mA.
OUT
SENSE
Input Bypass Capacitors
Capacitor Selection
TH
TH
. For high-temperature
Output Filter Capacitor
) of 1V.
of less than 2V. Figure
Reference Capacitor
, and that its break-
Diode Selection