Low-Cost, High-Frequency, Current-Mode PWM
Buck Controller
The chosen inductor’s saturation current rating ꢀust
exceed the expected peak inductor current (IPEAK).
Deterꢀine IPEAK as:
FET. A good general rule is to allow 0.5% additional
resistance for each °C of MOSFET junction teꢀperature
rise. The calculated V
ꢀust be less than V
.
CS
VALLEY
For the MAX1953, connect ILIM to GND for a short-
circuit current-liꢀit voltage of 105ꢀV, to V for 320ꢀV
IN
LIR
2
I
= I
+
× I
LOAD MAX
PEAK LOAD MAX
(
)
(
)
or leave ILIM floating for 210ꢀV.
MOSFET Selection
The MAX1953/MAX1954/MAX1957 drive two external,
logic-level, N-channel MOSFETs as the circuit switch
eleꢀents. The key selection paraꢀeters are:
Setting the Current Limit
The MAX1953/MAX1954/MAX1957 use a lossless cur-
rent-sense ꢀethod for current liꢀiting. The voltage
drops across the MOSFETs created by their on-resis-
tances are used to sense the inductor current.
Calculate the current-liꢀit threshold as follows:
• On-Resistance (R
): The lower, the better.
DS(ON)
• Maximum Drain-to-Source Voltage (V
): Should
DSS
be at least 20% higher than the input supply rail at
0.8V
the high side MOSFET’s drain.
V
=
CS
A
• Gate Charges (Q , Q , Q ): The lower, the better.
CS
g
gd
gs
For a 3.3V input application, choose a MOSFET with a
rated R at V = 2.5V. For a 5V input application,
where A
CS
is the gain of the current-sense aꢀplifier.
CS
DS(ON)
GS
A
is 6.3 for the MAX1953 when ILIM is connected to
choose the MOSFETs with rated R
at V
≤ 4.5V.
DS(ON)
GS
GND and 3.5 for the MAX1954/MAX1957, and for the
MAX1953 when ILIM is connected to IN or floating. The
0.8V is the usable dynaꢀic range of COMP (V
For a good coꢀproꢀise between efficiency and cost,
choose the high-side MOSFET (N1) that has conduction
losses equal to switching loss at the noꢀinal input volt-
age and output current. The selected low-side and high-
side MOSFETs (N2 and N1, respectively) ꢀust have
).
COMP
Initially, the high-side MOSFET is ꢀonitored. Once the
voltage drop across the high-side MOSFET exceeds V
,
CS
the high-side MOSFET is turned off and the low-side
MOSFET is turned on. The voltage across the low-side
MOSFET is then ꢀonitored. If the voltage across the low-
side MOSFET exceeds the short-circuit current liꢀit, a
short-circuit condition is deterꢀined and the low-side
MOSFET is held on. Once the ꢀonitored voltage falls
below the short-circuit current-liꢀit threshold, the
MAX1953/MAX1954/MAX1957 switch norꢀally. The short-
circuit current-liꢀit threshold is fixed at 210ꢀV for the
MAX1954/ MAX1957 and is selectable for the MAX1953.
R
that satisfies the current-liꢀit setting condition
DS(ON)
above. For N2, ꢀake sure 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 /Q ratio have higher iꢀꢀu-
gd gs
nity to dV/dt.
For proper therꢀal ꢀanageꢀent design, the power dis-
sipation ꢀust be calculated at the desired ꢀaxiꢀuꢀ
operating junction teꢀperature, T
. N1 and N2
J(MAX)
have different loss coꢀponents due to the circuit oper-
ation. N2 operates as a zero-voltage switch; therefore,
When selecting the high-side MOSFET, use the follow-
ing ꢀethod to verify that the MOSFET’s R
is suffi-
DS(ON)
ꢀajor losses are the channel conduction loss (P
)
N2CC
ciently low at the operating junction teꢀperature (T ):
J
and the body diode conduction loss (P
):
N2DC
0.8V
× I
PEAK
USER
AT T
J(MAX)
R
≤
DS(ON)
DS(ON)N1
A
CS
V
2
OUT
P
= (1−
) × I
× R
LOAD
N2CC
DS(ON)
The voltage drop across the low-side MOSFET at the
V
IN
valley point and at I
is:
LOAD(MAX)
P
= 2 × I
× V × t
× f
DT S
N2DC
LOAD
F
LIR
2
where V is the body diode forward-voltage drop, t is
F
dt
V
=R
× (I
−
× I
)
VALLEY
DS(ON)
LOAD(MAX)
LOAD MAX
(
)
the dead tiꢀe between N1 and N2 switching transi-
tions, and f is the switching frequency.
S
where R
is the ꢀaxiꢀuꢀ value at the desired
ꢀaxiꢀuꢀ operating junction teꢀperature of the MOS-
DS(ON)
16 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
