MAX8576 MAXIM [Maxim Integrated Products], MAX8576 Datasheet - Page 14

• Manufacturer Part Number: MAX8576
• Description: 3V to 28V Input, Low-Cost, Hysteretic Synchronous Step-Down Controllers
• Manufacturer: MAXIM [Maxim Integrated Products]

3V to 28V Input, Low-Cost, Hysteretic
Synchronous Step-Down Controllers
where V
output inductor, I
rent. The term 0.01V is to reflect 1/2 of the feedback-
threshold hysteresis.
The inductor value is bounded by two operating para-
meters: the switching frequency and the inductor peak-
to-peak ripple current. The peak-to-peak ripple current
is typically in the range of 20% to 40% of the maximum
output current. The equation below defines the induc-
tance value:
where LIR is the ratio of inductor current ripple to DC
load current and f
compromise between size, efficiency, and cost is an
LIR of 30%. The selected inductor must have a saturat-
ed current rating above the sum of the maximum output
current and half of the peak-to-peak ripple current. The
DC current rating of the inductor must be above the
maximum output current to keep the temperature rise
within the desired range. In addition, the DC resistance
of the inductor must meet the requirement below:
where V
drop from no load to full load (I
Resistor R2 (R7 for the MAX8577/MAX8579) of Figure 2
(Figure 3 for the MAX8577/MAX8579) sets the current
limit and is connected between OCSET and the drain of
the high-side n-channel MOSFET. An internal 50µA
current sink sets the maximum voltage drop across the
high-side n-channel MOSFET relative to V
mum V
lated by:
I
peak inductor current at the maximum output current.
Use R
The current limit is temperature compensated.
14
R
DS(MAX)
3
______________________________________________________________________________________
R
1
DS(ON)MAX
DS
FB
L
V
must be equal or greater than the maximum
OUT
DS ON MAX
drop needs to be determined. This is calcu-
V
= 0.590V, R
OUT
(
V
is the maximum-allowed output-voltage
IN
)
OUTMAX
V
at the junction temperature of +25 C.
S
OUT
0 01
R
is the switching frequency. A good
.
f
S
DC
V
Setting the Current Limit
I
DC
DS MAX
I
LOAD MAX
I
is the maximum output cur-
(
V
OUTMAX
is the DC resistance of the
R
IN
V
V
DC
FB
OUT
OUTMAX
)
V
OUT
0 5
R
Inductor Value
.
DS ON MAX
).
LIR
(
I
OUTMAX
IN
)
. The maxi-
1
R
following formula:
A 0.01µF ceramic capacitor is required in parallel with
R
The MAX8576–MAX8579 drive two external, logic-level,
n-channel MOSFETs as the circuit switching elements.
The key selection parameters are:
1) On-resistance (R
2) Maximum drain-to-source voltage (V
3) Gate charges (Q
For a 3.3V input application, choose a MOSFET with a
rated R
tion, choose the MOSFETs with rated R
cost, choose the high-side MOSFET (N1) that has con-
duction losses equal to switching loss at nominal input
voltage and output current. The selected high-side
MOSFET (N1) must have R
rent-limit-setting condition above. For N2, make sure
that it does not spuriously turn on due to dV/dt caused
by N1 turning on as this results in shoot-through current
degrading the efficiency. MOSFETs with a lower Q
Q
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
MOSFET, it could be either at V
and N2 have different loss components due to the cir-
cuit operation. N2 operates as a zero-voltage switch;
therefore, major losses are: the channel-conduction
loss (P
(P
Use R
where V
the dead time between N1 and N2 switching transitions
(40ns typ), and f
OCSET
OCSET
gs
4.5V. For a good compromise between efficiency and
N2DC
be at least 20% higher than the input supply rail at
the high-side MOSFET’s drain.
ratio have higher immunity to dV/dt.
DS(ON)
).
DS(ON)
N2CC
F
P
to decouple high-frequency noise.
is calculated using the V
N CC
is the body-diode forward-voltage drop, t
2
P
N DC
2
) and the body-diode conduction loss
at T
at V
R
S
J(MAX)
OCSET
1
is the switching frequency.
GS
2
g
DS(ON)
V
, Q
OUT
V
= 2.5V. For a 5V input applica-
IN
.
I
gd
LOAD
V
, Q
DS(ON)
): the lower, the better.
DS ON MAX
IN(MAX)
gs
50
(
I
LOAD
MOSFET Selection
IN(MAX)
): the lower, the better.
V
F
)
A
that satisfies the cur-
DS(ON)MAX
2
t
; for the high-side
dt
or V
R
DS(ON)
f
S
DSS
DS ON
IN(MIN)
(
): should
with the
)
at V
). N1
DT
gd
GS
is
/