Datasheet

MAX1953/MAX1954/MAX1957
Low-Cost, High-Frequency, Current-Mode PWM
Buck Controller
______________________________________________________________________________________ 17
N1 operates as a duty-cycle control switch and has the
following major losses: the channel conduction loss
(P
N1CC
), the voltage and current overlapping switching
loss (P
N1SW
), and the drive loss (P
N1DR
).
where I
GATE
is the average DH driver output current
capability determined by:
where R
DH
is the high-side MOSFET driver’s on-resis-
tance (3Ω max) and R
GATE
is the internal gate resis-
tance of the MOSFET (~ 2Ω):
where V
GS
~ V
IN
. In addition to the losses above, allow
about 20% more for additional losses due to MOSFET
output capacitances and N2 body diode reverse recov-
ery charge dissipated in N1 that exists, but is not well
defined in the MOSFET data sheet. Refer to the MOS-
FET data sheet for the thermal-resistance specification
to calculate the PC board area needed to maintain the
desired maximum operating junction temperature with
the above calculated power dissipations.
The minimum load current must exceed the high-side
MOSFET’s maximum leakage current over temperature
if fault conditions are expected.
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor must meet the ripple current
requirement (I
RMS
) imposed by the switching currents
defined by the following equation:
I
RMS
has a maximum value when the input voltage
equals twice the output voltage (V
IN
= 2 x V
OUT
), where
I
RMS(MAX)
= I
LOAD
/2. Ceramic capacitors are recom-
mended due to their low ESR and ESL at high frequency,
with relatively low cost. Choose a capacitor that exhibits
less than 10°C temperature rise at the maximum operat-
ing RMS current for optimum long-term reliability.
Output Capacitor
The key selection parameters for the output capacitor
are the actual capacitance value, the equivalent series
resistance (ESR), the equivalent series inductance
(ESL), and the voltage-rating requirements. These para-
meters affect the overall stability, output voltage ripple,
and transient response. The output ripple has three
components: variations in the charge stored in the out-
put capacitor, the voltage drop across the capacitor’s
ESR, and the voltage drop across the ESL caused by
the current into and out of the capacitor:
The output voltage ripple as a consequence of the ESR,
ESL, and output capacitance is:
where I
P-P
is the peak-to-peak inductor current (see the
Determining the Inductor Value section). These equa-
tions are suitable for initial capacitor selection, but final
values should be chosen based on a prototype or eval-
uation circuit.
As a general rule, a smaller current ripple results in less
output voltage ripple. Since the inductor ripple current
is a factor of the inductor value and input voltage, the
output voltage ripple decreases with larger inductance,
and increases with higher input voltages. Ceramic
capacitors are recommended for the MAX1953 due to
its 1MHz switching frequency. For the MAX1954/
MAX1957, using polymer, tantalum, or aluminum elec-
trolytic capacitors is recommended. The aluminum
electrolytic capacitor is the least expensive; however, it
has higher ESR. To compensate for this, use a ceramic
capacitor in parallel to reduce the switching ripple and
noise. For reliable and safe operation, ensure that the
capacitor’s voltage and ripple-current ratings exceed
the calculated values.
V I ESR
V
I
Cf
V
V
L
ESL
I
VV
fL
V
V
RIPPLE
ESR
PP
RIPPLE C
PP
OUT
S
RIPPLE ESL
IN
PP
IN OUT
S
OUT
IN
()
()
()
=×
××
⎛
⎝
⎜
⎞
⎠
⎟
=
−
×
⎛
⎝
⎜
⎞
⎠
⎟
×
⎛
⎝
⎜
⎞
⎠
⎟
−
−
=
−
8
VV V V
RIPPLE RIPPLE
ESR
RIPPLE C RIPPLE ESL
=++
()
() ( )
I
IVVV
V
RMS
LOAD OUT IN OUT
IN
=
××−
()
PQVf
R
RR
NDR G GS
S
GATE
GATE DH
1
=× ××
+
I
V
RR
GATE
IN
DH GATE
≅ ×
+
1
2
P
V
V
I R USE R AT T
PVI
QQ
I
f
NCC
OUT
IN
LOAD
DS ON DS ON J MAX
N SW IN LOAD
GS GD
GATE
S
1
2
2
=
⎛
⎝
⎜
⎞
⎠
⎟
××
()
=× ×
+
⎛
⎝
⎜
⎞
⎠
⎟
×
() () ( )