MAX1954AEUB-T Maxim Integrated Products, MAX1954AEUB-T Datasheet - Page 13

• Manufacturer Part Number: MAX1954AEUB-T
• Description: Current Mode PWM Controllers
• Manufacturer: Maxim Integrated Products

Specifications of MAX1954AEUB-T

• Number Of Outputs: 1
• Duty Cycle (max): 93 %
• Output Voltage: 0.8 V to 4.95 V
• Output Current: 25000 mA
• Mounting Style: SMD/SMT
• Package / Case: uSOP-10
• Switching Frequency: 360 KHz
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Synchronous Pin: No
• Topology: Boost, Buck
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

choose the high-side MOSFET (N1) that has conduction
losses equal to switching loss at nominal input voltage
and output current. The selected MOSFETs must have an
R
above. For N2, ensure that it does not spuriously turn on
due to dV/dt caused by N1 turning on, as this would
result in shoot-through current degrading the efficiency.
MOSFETs with a lower Q
ty to dV/dt.
For proper thermal-management design, the power dis-
sipation must be calculated at the desired maximum
operating junction temperature, T
have different loss components due to the circuit oper-
ation. N2 operates as a zero-voltage switch; therefore,
major losses are the channel-conduction loss (P
and the body-diode conduction loss (P
where V
the dead time between N1 and N2 switching transitions,
f
N1 operates as a duty-cycle control switch and has the
following major losses: the channel-conduction loss
(P
and the drive loss (P
diode conduction loss, because the diode never con-
ducts current.
where I
capability determined by:
where R
on-resistance (1.5
resistance of the MOSFET (~2 ).
where V
Use R
Use R
S
V
DS(ON)
N1CC
VALLEY
is the switching frequency, and t
P
N DR
1
DS ON
DS ON
), the VL overlapping switching loss (P
GATE
GS
F
(
P
(
DS(ON)(N2)
that satisfies the current-limit setting condition
P
N CC
is the body-diode forward-voltage drop, t
N SW
I
~V
1
Q
GATE
R
1
)
)
DS ON
at T
g
P
IN.
at T
is the average DH-driver output current
N DC
(
2
J MAX
V
(
J MAX
V
GS
V
)
(
______________________________________________________________________________________
IN
OUT
V
IN
typ) and R
is the high-side MOSFET driver’s
0 5
)
.
N1DR
2
.
(
Controller with Foldback Current Limit
I
)
LOAD MAX
.
gd
I
f
S
LOAD
/Q
I
LOAD
). N1 does not have body-
R
I
(
2
gs
DS ON N
R
LOAD
Low-Cost, Current-Mode PWM Buck
GATE
ratio have higher immuni-
GATE
(
)
dt
Q
)(
J(MAX)
V
LIR
is 20ns (typ).
gs
F
R
is the internal gate
V
2
2
R
I
IN
GATE
GATE
)
N2DC
R
DS ON
DS ON N
t
(
dt
. N1 and N2
Q
R
(
gd
).
I
GATE
LOAD MAX
)
)(
f
S
N1SW
2
N2CC
)
dt
f
S
is
),
)
In addition to the losses above, allow approximately
20% for additional losses due to MOSFET output capac-
itances 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 maxi-
mum operating junction temperature with the above cal-
culated power dissipations.
To reduce electromagnetic interference (EMI) caused
by switching noise, add a 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 resis-
tors 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 overtemperature
if fault conditions are expected.
Fast-switching transitions cause ringing because of
resonating circuit parasitic inductance and capaci-
tance at the switching nodes. This high-frequency ring-
ing occurs at LX’s rising and falling transitions and can
interfere with circuit performance and generate EMI. To
dampen this ringing, a series RC snubber circuit is
added across each switch. Below is the procedure for
selecting the value of the series RC circuit:
1) Connect a scope probe to measure the voltage
2) Find the capacitor value (connected from LX to
The circuit parasitic capacitance (C
equal to 1/3rd of the value of the added capacitance
above. The circuit parasitic inductance (L
ed by:
The resistor for critical dampening (R
2 x f
tailor the desired damping and the peak voltage excur-
sion. The capacitor (C
four times the value of the C
power loss of the snubber circuit (P
ed in the resistor R
from LX to GND, and observe the ringing frequen-
cy, f
GND) that reduces the ringing frequency by half.
R
x L
R
.
PAR
P
RSNUB
. Adjust the resistor value up or down to
L
PAR
SNUB
SNUB
MOSFET Snubber Circuit
2
C
and can be calculated as:
SNUB
f
R
) should be at least two to
PAR
2
1
C
to be effective. The
V
PAR
RSNUB
IN
PAR
SNUB
2
PAR
) at LX is then
) is dissipat-
) is equal to
f
) is calculat-
S
13