Datasheet

MAX8650
4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency
______________________________________________________________________________________ 19
The DC resistance of the inductor’s copper wire has a
+0.22%/°C temperature coefficient.
To use the DC resistance of the output inductor for cur-
rent sensing, an RC circuit is added (see Figure 8). The
RC time constant is set at twice the inductor (L/R
DC
) time
constant. Pick the value of C9 (typically 0.47µF), then cal-
culate the resistor value from R4 = 2L / (R
DC
x C9).
Add a resistor (R5 in Figure 8) to the CS- connection to
minimize input offset error. Calculate the value of R5 as
follows:
1) When V
OUT
≥ 2.4V:
2) When V
OUT
< 2.4V:
Capacitor C13 is connected in parallel with R5 and is
equal in value to C9.
The equivalent current-sense resistance when using an
inductor for current sensing is equal to the DC resis-
tance of the inductor (R
DC
).
MOSFET Selection
The MAX8650 drives two or four external, logic-level, n-
channel MOSFETs as the circuit switch elements. The
key selection parameters are:
1) On-resistance (R
DS(ON)
): the lower, the better.
2) Maximum drain-to-source voltage (V
DSS
): should
be at least 20% higher than the input supply rail at
the high-side MOSFET’s drain.
3) Gate charges (Q
G
, Q
GD
, Q
GS
): the lower, the better.
For a 5V input application, choose the MOSFETs with
rated R
DS(ON)
at V
GS
≤ 4.5V. With higher input volt-
ages, the internal VL regulator provides 6.5V for gate
drive to minimize the on-resistance for a wide range of
MOSFETs.
For a good compromise between efficiency and cost,
choose the high-side MOSFET (N1, N2) that has con-
duction losses equal to switching losses at nominal
input voltage and output current. The selected low-side
MOSFET (N3, N4) must have an R
DS(ON)
that satisfies
the current-limit-setting condition above. Make sure that
the low-side MOSFET does not spuriously turn on due
to dV/dt caused by the high-side MOSFET turning on,
as this would result in shoot-through current and
degrade the efficiency. MOSFETs with a lower
Q
GD
/Q
GS
ratio have higher immunity to dV/dt. For high-
current applications, it is often preferable to parallel two
MOSFETs rather than to use a single large MOSFET.
For proper thermal-management design, the power dis-
sipation must be calculated at the desired maximum
operating junction temperature, maximum output cur-
rent, and worst-case input voltage (for the low-side
MOSFET, worst case is at V
IN(MAX)
; for the high-side
MOSFET, it could be either at V
IN(MAX)
or V
IN(MIN)
).
The high-side and low-side MOSFETs have different
loss components due to the circuit operation. The low-
side MOSFET operates as a zero voltage switch; there-
fore, major losses are the channel-conduction loss
(P
LSCC
) and the body-diode conduction loss (P
LSDC
).
R
AxR
A
RA
k
ILIM
5
15 4
15
10
32
1
=
+
×
⎛
⎝
⎜
⎞
⎠
⎟
μ
μ
μ
Ω
R
A
RA
k
R
A
ILIM
5
20
10
32
4
20
1
=
+
×
Ω
⎛
⎝
⎜
⎞
⎠
⎟
×μ
μ
μ
MAX8650
CS-
LX
R4
R5 C13
L1
C9
CS+
V
OUT
Figure 8. Current Sense Using the Inductor’s DC Resistance
MAX8650
CS-
LX
R4
R5
C13
L1
C9
CS+
V
OUT
R3
Figure 9. Using a Current-Sense Resistor for Improved Current-
Sense Accuracy