MAX1954A
Low-Cost, Current-Mode PWM Buck
Controller with Foldback Current Limit
12 ______________________________________________________________________________________
Shutdown
The MAX1954A features a low-power shutdown mode.
Use an open-collector, NPN transistor to pull COMP low
and shut down the IC. COMP must be pulled below
0.25V to shut down the MAX1954A. Choose a transistor
with a V
CE(SAT)
below 0.25V. During shutdown, the out-
put is high impedance. Shutdown reduces the quies-
cent current (I
Q
) to 220µA (typ). Note that implementing
shutdown in this fashion discharges the output only
until the inductor runs out of energy. Upon recovery,
soft-start is not available. Only the foldback current limit
results in pseudo-soft-start mode.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipa-
tion in the MAX1954A. When the junction temperature
exceeds T
J
= +160°C, an internal thermal sensor shuts
down the IC, allowing the IC to cool. The thermal sensor
turns the IC on again after the junction temperature
cools by 15°C, resulting in a pulsed output during con-
tinuous thermal-overload conditions.
Design Procedures
Setting the Output Voltage
To set the output voltage for the MAX1954A, connect
FB to the center of an external resistor-divider from the
output to GND (Figures 1 and 2). Select R2 between
8kΩ and 24kΩ, and calculate R1 by:
where V
FB
= 0.8V. R1 and R2 should be placed as
close as possible to the IC.
Inductor Value
There are several parameters that must be examined
when determining which inductor to use. Input voltage,
output voltage, load current, switching frequency, and
LIR. LIR is the ratio of inductor current ripple to DC load
current. A higher LIR value allows for a smaller induc-
tor, but results in higher losses and higher output rip-
ple. A good compromise between size and efficiency is
an LIR of 30%. Once all of the parameters are chosen,
the inductor value is determined as follows:
where f
S
is the switching frequency. Choose a standard
value close to the calculated value. The exact inductor
value is not critical and can be adjusted to make trade-
offs among size, cost, and efficiency. Lower inductor val-
ues minimize size and cost, but they also increase the
output ripple and reduce the efficiency due to higher
peak currents. On the other hand, higher inductor values
increase efficiency, but eventually resistive losses, due to
extra turns of wire, exceed the benefit gained from lower
AC levels. Find a low-loss inductor with the lowest possi-
ble DC resistance that fits the allotted dimensions. Ferrite
cores are often the best choice. However, powdered iron
is inexpensive and can work well at 300kHz. The chosen
inductor’s saturation current rating must exceed the peak
inductor current determined as:
Setting the Current Limit
The MAX1954A uses a valley current-sense method for
current limiting. The voltage drop across the low-side
MOSFET due to its on-resistance is used to sense the
inductor current. The voltage drop across the low-side
MOSFET at the valley point and at I
LOAD(MAX)
is:
The calculated V
VALLEY
must be less than the minimum
current-limit threshold specified.
Additionally, the high-side MOSFET R
DS(ON)
must meet
the following equation to avoid tripping the internal
peak-current clamp circuit prematurely:
R
DS(ON)
< 0.8V / (3.65 x (I
LOAD(MAX)
x ( 1 + LIR / 2)))
Use the maximum R
DS(ON)
value at the desired maxi-
mum operating junction temperature of the MOSFET. A
good general rule is to allow 0.5% additional resistance
for each °C of MOSFET junction-temperature rise.
MOSFET Selection
The MAX1954A drives two external, logic-level, N-chan-
nel MOSFETs as the circuit-switch elements. The key
selection parameters are:
1) On-resistance (R
DS(ON)
): the lower, the better.
However, the current-sense signal (R
DS
x I
PEAK
)
must be greater than 16mV at maximum load.
2) Maximum drain-to-source voltage (V
DSS
): it should
be at least 20% higher than the input supply rail at
the high-side MOSFET’s drain.
3) 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,
