Datasheet
MAX8576–MAX8579
3V to 28V Input, Low-Cost, Hysteretic
Synchronous Step-Down Controllers
______________________________________________________________________________________ 15
N1 operates as a duty-cycle control switch and has the
following major losses: the channel-conduction loss
(P
N1CC
), the VL overlapping switching loss (P
N1SW
),
and the drive loss (P
N1DR
). N1 does not have body-
diode conduction loss because the diode never con-
ducts current.
Use R
DS(ON)
at T
J(MAX)
.
where I
GATE
is the average DH driver output-current
capability determined by:
where R
DH
is the high-side MOSFET driver’s on-resis-
tance (2Ω typ) and R
GATE
is the internal gate resis-
tance of the MOSFET (approximately 2Ω).
where V
GS
is approximately equal to V
L.
In addition to the losses above, allow about 20% more
for additional losses due to MOSFET output capaci-
tances and N2 body-diode reverse-recovery charge
dissipated in N1 that exists, but is not well defined in
the MOSFET data sheet. Refer to the MOSFET data
sheet for thermal-resistance specification to calculate
the PC board area needed to maintain the desired max-
imum operating junction temperature with the above
calculated power dissipations.
To reduce EMI caused by switching noise, add 0.1µF
ceramic capacitor from the high-side switch drain to the
low-side switch source or add resistors in series with
DH and DL to slow down the switching transitions.
However, adding series resistors increases the power
dissipation of the MOSFET, so be sure this does not
overheat the MOSFET.
The minimum load current must exceed the high-side
MOSFET’s maximum leakage current over temperature
if fault conditions are expected.
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor must meet the ripple-current
requirement (I
RMS
) imposed by the switching currents
defined by the following equation:
I
RMS
has a maximum value when the input voltage
equals twice the output voltage (V
IN
= 2 x V
OUT
), so
I
RMS(MAX)
= I
LOAD
/ 2. Ceramic capacitors are recom-
mended due to their low ESR and ESL at high frequen-
cy, with relatively lower cost. Choose a capacitor that
exhibits less than 10°C temperature rise at the maximum
operating RMS current for optimum long-term reliability.
Output Capacitor
The key selection parameters for the output capacitor
are the actual capacitance value, the ESR, the equiva-
lent series inductance (ESL), and the voltage-rating
requirements. These parameters affect the overall sta-
bility, output voltage ripple, and transient response. The
output ripple has three components: variations in the
charge stored in the output capacitor, the voltage drop
across the capacitor’s ESR, and the ESL caused by the
current into and out of the capacitor. The maximum out-
put ripple voltage can be estimated by:
The output voltage ripple as a consequence of the ESR
and output capacitance is:
where I
P-P
is the peak-to-peak inductor current (see the
Inductor Value section). These equations are suitable
for initial capacitor selection, but final values should be
I
VV
fL
V
V
PP
IN OUT
S
OUT
IN
−
=
−
×
⎛
⎝
⎜
⎞
⎠
⎟
×
⎛
⎝
⎜
⎞
⎠
⎟
V
V
L
ESL
RIPPLE ESL
IN
()
=
⎛
⎝
⎜
⎞
⎠
⎟
×
V
I
Cf
RIPPLE C
PP
OUT
S
()
=
×
−
V I ESR
RIPPLE
ESR
PP
()
=×
−
VV V V
RIPPLE RIPPLE
ESR
RIPPLE C RIPPLE ESL
=++
()
() ( )
I
IVVV
V
RMS
LOAD OUT IN OUT
IN
=
××−
()
PQVf
R
RR
NDR g GS
S
GATE
GATE DH
1
=× ××
+
I
V
RR
GATE
L
DH GATE
.
≅×
+
05
PVI
QQ
I
f
N SW IN LOAD
gs gd
GATE
S
1
=× ×
+
⎛
⎝
⎜
⎞
⎠
⎟
×
P
V
V
IR
NCC
OUT
IN
LOAD DS ON1
2
=
⎛
⎝
⎜
⎞
⎠
⎟
××
()