MAX767 Maxim, MAX767 Datasheet - Page 14

• Manufacturer Part Number: MAX767
• Description: 5V-to-3.3V / Synchronous / Step-Down Power-Supply Controller
• Manufacturer: Maxim

Proper circuit operation requires that the short-circuit
current be at least I
standard application circuits are designed for a short-
circuit current slightly in excess of this amount. This
excess design current guarantees proper start-up
under constant full-load conditions and proper full-load
transient response, and is particularly necessary with
low input voltages. If the circuit will not be subjected to
full-load transients or to loads approaching the full-load
at start-up, you can decrease the short-circuit current
by increasing R1, as described in the Current-Sense
Resistor section. This may allow use of MOSFETs with a
lower current-handling capability.
Losses due to parasitic resistances in the switches,
coil, and sense resistor dominate at high load-current
levels. Under heavy loads, the MAX767 operates deep
in the continuous-conduction mode, where there is a
large DC offset to the inductor current (plus a small
sawtooth AC component) (see Inductor section). This
DC current is exactly equal to the load current, a fact
which makes it easy to estimate resistive losses via the
simplifying assumption that the total inductor current is
equal to this DC offset current. The major loss mecha-
nisms under heavy loads, in usual order of importance,
are:
Inductor-core losses, which are fairly low at heavy
loads because the AC component of the inductor cur-
rent is small, are not accounted for in this analysis.
5V-to-3.3V, Synchronous, Step-Down
Power-Supply Controller
14
I
gate-charge losses
diode-conduction losses
transition losses
capacitor-ESR losses
losses due to the operating supply current of the IC.
2
R losses
______________________________________________________________________________________
Efficiency = ______ x 100% =
PD
TOTAL
= PD
PD
LOAD
P
P
_______________ x 100%
P
(I
TRAN
OUT
IN
OUT
2
R)
P
+ PD
x (1 + LIR / 2). However, the
OUT
+ PD
+ PD
Heavy-Load Efficiency
GATE
TOTAL
CAP
+ PD
+ PD
DIODE
IC
+
where R
r
FET. Note that the r
MOSFETs are employed for both the synchronous recti-
fier and high-side switch, because they time-share the
inductor current. If the MOSFETs are not identical, esti-
mate losses by averaging the two individual r
terms according to their duty factors: 0.66 for N1 and
0.34 for N2.
where q
high-side switches. Note that gate-charge losses are
dissipated in the IC, not the MOSFETs, and therefore
contribute to package temperature rise. For a pair of
matched MOSFETs, q
tance of a single MOSFET (a data sheet specification).
where V
at the output current, t
(typically 110ns), and f is the switching frequency.
where C
high-side MOSFET (a data sheet parameter), f is the
switching frequency, and I
available from the high-side gate driver output (approx-
imately 1A).
Additional switching losses are introduced by other
sources of stray capacitance at the switching node,
including the catch-diode capacitance, coil interwind-
ing capacitance, and low-side switch drain capaci-
tance, and are given as PD
these are usually negligible compared to C
The low-side switch introduces only tiny switching loss-
es, since its drain-source voltage is already low when it
turns on.
DS(ON)
PD
PD
PD
PD
D
is the drain-source on resistance of the MOS-
G
RSS
(I
GATE
DIODE
TRAN
COIL
2
is the forward voltage of the Schottky diode
is the sum of the gate charge for low- and
R)
is the reverse transfer capacitance of the
= resistive loss = (I
= gate driver loss = q
= transition loss =
is the DC resistance of the coil and
= diode conduction losses =
(R
______________________
V
I
COIL
LOAD
IN
DS(ON)
2
G
x C
D
+ r
is simply twice the gate capaci-
x V
is the diode’s conduction time
I
RSS
DS(ON)
DRIVE
SW
term assumes that identical
D
Diode Conduction Losses
DRIVE
x t
x I
= V
D
LOAD
LOAD
+ R1)
Gate-Charge Losses
x f
IN
is the peak current
G
Transition Losses
2
2
x f
x C
x f x 5V
) x
STRAY
I
RSS
2
R Losses
DS(ON)
x f, but
losses.