1.8V to 28V Input, PWM Step-Up
Controllers in µMAX
given output ripple. An inductance value larger than
may also be used, but output-filter capacitance
old NFETs that specify on-resistance with a gate-
L
source voltage (V ) of 2.7V or less. When selecting an
GS
IDEAL
must be increased by the same proportion that L has to
NFET, key parameters can include:
L
. See the Capacitor Selection section for more
IDEAL
1) Total gate charge (Q )
g
information on determining output filter values.
2) Reverse transfer capacitance or charge (C
)
RSS
Due the MAX668/MAX669’s high switching frequencies,
inductors with a ferrite core or equivalent are recom-
mended. Powdered iron cores are not recommended
due to their high losses at frequencies over 50kHz.
3) On-resistance (R
)
DS(ON)
4) Maximum drain-to-source voltage (V
)
DS(MAX)
5) Minimum threshold voltage (V
)
TH(MIN)
At high switching rates, dynamic characteristics (para-
meters 1 and 2 above) that predict switching losses
Determining Peak Inductor Current
The peak inductor current required for a particular out-
put is:
may have more impact on efficiency than R
DS(ON),
which predicts DC losses. Q includes all capacitances
g
I
= I
+ (I
/ 2)
LPEAK
LDC
LPP
associated with charging the gate. In addition, this
parameter helps predict the current needed to drive the
gate at the selected operating frequency. The continu-
ous LDO current for the FET gate is:
where I
is the average DC input current and I
is
LDC
LPP
and
the inductor peak-to-peak ripple current. The I
LDC
8/MAX69
I
terms are determined as follows:
LPP
I
(V
+ V )
OUT OUT D
I
= Q x f
g OSC
GATE
I
=
LDC
(V – V
)
IN
SW
For example, the MMFT3055L has a typical Q of 7nC
g
(at V = 5V); therefore, the I
current at 500kHz is
3.5mA. Use the FET manufacturer’s typical value for Q
GS
GATE
where V is the forward voltage drop across the
D
g
Schottky rectifier diode (D1), and V
across the external FET, when on.
is the drop
SW
in the above equation, since a maximum value (if sup-
plied) is usually too conservative to be of use in esti-
mating I
.
(V – V ) (V
+ V – V )
GATE
IN
SW
OUT
(V
D
IN
I
=
LPP
L x f
+ V )
Diode Selection
OSC
OUT
D
where L is the inductor value. The saturation rating of
The MAX668/MAX669’s high switching frequency
demands a high-speed rectifier. Schottky diodes are
recommended for most applications because of their
fast recovery time and low forward voltage. Ensure that
the diode’s average current rating is adequate using
the diode manufacturer’s data, or approximate it with
the following formula:
the selected inductor should meet or exceed the calcu-
lated value for I , although most coil types can be
LPEAK
operated up to 20% over their saturation rating without
difficulty. In addition to the saturation criteria, the induc-
tor should have as low a series resistance as possible.
For continuous inductor current, the power loss in the
inductor resistance, P , is approximated by:
LR
I
- I
LPEAK
OUT
I
= I
+
2
P
LR
(I
x V
/ V ) x R
DIODE
OUT
OUT
OUT
IN
L
3
where R is the inductor series resistance.
L
Also, the diode reverse breakdown voltage must
exceed V . For high output voltages (50V or above),
Once the peak inductor current is selected, the current-
sense resistor (R ) is determined by:
OUT
CS
Schottky diodes may not be practical because of this
voltage requirement. In these cases, use a high-speed
silicon rectifier with adequate reverse voltage.
R
= 85mV / I
LPEAK
CS
For high peak inductor currents (>1A), Kelvin sensing
connections should be used to connect CS+ and
Capacitor Selection
PGND to R . PGND and GND should be tied together
CS
Output Filter Capacitor
The minimum output filter capacitance that ensures sta-
bility is:
at the ground side of R
.
CS
Power MOSFET Selection
The MAX668/MAX669 drive a wide variety of N-channel
power MOSFETs (NFETs). Since LDO limits the EXT
output gate drive to no more than 5V, a logic-level
NFET is required. Best performance, especially at low
input voltages (below 5V), is achieved with low-thresh-
(7.5V x L / L
)
IDEAL
x f
C
=
OUT(MIN)
(2πR
x V
)
OSC
CS
IN(MIN)
where V
is the minimum expected input voltage.
OUT(MIN)
IN(MIN)
Typically C
, though sufficient for stability, will
14 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
