Datasheet

MAX8576–MAX8579
3V to 28V Input, Low-Cost, Hysteretic
Synchronous Step-Down Controllers
14 ______________________________________________________________________________________
where V
FB
= 0.590V, R
DC
is the DC resistance of the
output inductor, I
OUTMAX
is the maximum output cur-
rent. The term 0.01V is to reflect 1/2 of the feedback-
threshold hysteresis.
Inductor Value
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
S
is the switching frequency. A good
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 ΔV
OUT
is the maximum-allowed output-voltage
drop from no load to full load (I
OUTMAX
).
Setting the Current Limit
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
IN
. The maxi-
mum V
DS
drop needs to be determined. This is calcu-
lated by:
I
DS(MAX)
must be equal or greater than the maximum
peak inductor current at the maximum output current.
Use R
DS(ON)MAX
at the junction temperature of +25°C.
The current limit is temperature compensated.
R
OCSET
is calculated using the V
DS(ON)MAX
with the
following formula:
A 0.01µF ceramic capacitor is required in parallel with
R
OCSET
to decouple high-frequency noise.
MOSFET Selection
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
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 3.3V input application, choose a MOSFET with a
rated R
DS(ON)
at V
GS
= 2.5V. For a 5V input applica-
tion, choose the MOSFETs with rated R
DS(ON)
at V
GS
≤ 4.5V. For a good compromise between efficiency and
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
DS(ON)
that satisfies the cur-
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
gd
/
Q
gs
ratio have higher immunity to dV/dt.
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)
). N1
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
N2CC
) and the body-diode conduction loss
(P
N2DC
).
Use R
DS(ON)
at T
J(MAX)
.
where V
F
is the body-diode forward-voltage drop, t
DT
is
the dead time between N1 and N2 switching transitions
(40ns typ), and f
S
is the switching frequency.
PIVtf
N DC LOAD F dt S2
2=× ×××
P
V
V
IR
NCC
OUT
IN
LOAD DS ON2
2
1=−
⎛
⎝
⎜
⎞
⎠
⎟
××
()
R
V
A
OCSET
DS ON MAX
=
()
50μ
VIR
DS ON MAX DS MAX DS ON MAX() ( ) ()
=×
R
V
I
DC
OUT
OUTMAX
≤
Δ
L
VVV
V f I LIR
OUT IN OUT
IN S LOAD MAX
=
×−
()
×× ×
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
()
RR
VVR I
V
OUT DC OUTMAX
FB
31
001 05
1=×
++××
()
−
⎛
⎝
⎜
⎞
⎠
⎟
..