Datasheet

MAX1953/MAX1954/MAX1957
Low-Cost, High-Frequency, Current-Mode PWM
Buck Controller
16 ______________________________________________________________________________________
The chosen inductor’s saturation current rating must
exceed the expected peak inductor current (IPEAK).
Determine IPEAK as:
Setting the Current Limit
The MAX1953/MAX1954/MAX1957 use a lossless cur-
rent-sense method for current limiting. The voltage
drops across the MOSFETs created by their on-resis-
tances are used to sense the inductor current.
Calculate the current-limit threshold as follows:
where A
CS
is the gain of the current-sense amplifier.
A
CS
is 6.3 for the MAX1953 when ILIM is connected to
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 dynamic range of COMP (V
COMP
).
Initially, the high-side MOSFET is monitored. 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 monitored. If the voltage across the low-
side MOSFET exceeds the short-circuit current limit, a
short-circuit condition is determined and the low-side
MOSFET is held on. Once the monitored voltage falls
below the short-circuit current-limit threshold, the
MAX1953/MAX1954/MAX1957 switch normally. The short-
circuit current-limit threshold is fixed at 210mV for the
MAX1954/ MAX1957 and is selectable for the MAX1953.
When selecting the high-side MOSFET, use the follow-
ing method to verify that the MOSFET’s R
DS(ON)
is suffi-
ciently low at the operating junction temperature (T
J
):
The voltage drop across the low-side MOSFET at the
valley point and at I
LOAD(MAX)
is:
where R
DS(ON)
is the maximum value at the desired
maximum operating junction temperature of the MOS-
FET. A good general rule is to allow 0.5% additional
resistance for each °C of MOSFET junction temperature
rise. The calculated V
VALLEY
must be less than V
CS
.
For the MAX1953, connect ILIM to GND for a short-
circuit current-limit voltage of 105mV, to V
IN
for 320mV
or leave ILIM floating for 210mV.
MOSFET Selection
The MAX1953/MAX1954/MAX1957 drive two external,
logic-level, N-channel MOSFETs as the circuit switch
elements. The key selection parameters are:
• On-Resistance (R
DS(ON)
): The lower, the better.
• 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.
• 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 application,
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 conduction
losses equal to switching loss at the nominal input volt-
age and output current. The selected low-side and high-
side MOSFETs (N2 and N1, respectively) must have
R
DS(ON)
that satisfies the current-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 would
result in shoot-through current degrading the efficiency.
MOSFETs with a lower Q
gd
/Q
gs
ratio have higher immu-
nity to dV/dt.
For proper thermal management design, the power dis-
sipation must be calculated at the desired maximum
operating junction temperature, T
J(MAX)
. N1 and N2
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
N2CC
)
and the body diode conduction loss (P
N2DC
):
where V
F
is the body diode forward-voltage drop, t
dt
is
the dead time between N1 and N2 switching transi-
tions, and f
S
is the switching frequency.
USE R AT T
P
V
V
IR
PIVtf
DS ON J MAX
NCC
OUT
IN
LOAD
DS ON
N DC LOAD F DT S
() ( )
()
()
2
2
2
1
2
= − ××
=× × × ×
VR I
LIR
I
VALLEY DS ON LOAD MAX LOAD MAX
=× −
⎛
⎝
⎜
⎞
⎠
⎟
×
()
() ( )
()
2
R
V
AI
DS ON N
CS PEAK
()
.
1
08
≤
×
V
V
A
CS
CS
=
08.
II
LIR
I
PEAK LOAD MAX LOAD MAX
=+
⎛
⎝
⎜
⎞
⎠
⎟
×
() ()
2