MAX1636 Maxim, MAX1636 Datasheet - Page 21

• Manufacturer Part Number: MAX1636
• Description: Low-Voltage / Precision Step-Down Controller for Portable CPU Power
• Manufacturer: Maxim

where R
the MOSFET on-resistance, and R
sense resistor value. The R
cal MOSFETs for the high-side and low-side switches
because they time-share the inductor current. If the
MOSFETs are not identical, their losses can be estimat-
ed by averaging the losses according to duty factor.
where C
high-side MOSFET (a data-sheet parameter), I
the DH gate-driver peak output current (1.5A typ), and
20ns is the rise/fall time of the DH driver (20ns typ).
where VL is the internal logic-supply voltage (+5V), and
Q
and high-side switches. For matched MOSFETs, Q
twice the data-sheet value of an individual MOSFET. If
V
tion with V
improved by connecting VL to an efficient 5V source,
such as the system +5V supply.
where t
V
Table 6. Low-Voltage Troubleshooting Chart
OUT
FWD
Sag or droop in V
step-load change
Dropout voltage is too high
(V
decreases)
Unstable—jitters between
different duty factors and
frequencies
Poor efficiency
Won’t start under load or
quits before battery is
completely dead
g
PD(tran) = transition loss = V
OUT
is the sum of the gate-charge values for low-side
is set to less than 4.5V, replace VL in this equa-
is the forward voltage of the diode. This power is
D
follows V
DC
RSS
SYMPTOM
P(diode) = diode conduction losses =
is the diode-conduction time (120ns typ), and
is the DC resistance of the coil, R
BATT
is the reverse transfer capacitance of the
[(V
IN
IN
OUT
I
P(gate) = Q
as V
. In this case, efficiency can be
LOAD
______________________________________________________________________________________
C
RSS
under
IN
x V
/ I
FWD
GATE
DS(ON)
g
Low V
differential, <1.5V
Low V
differential, <1V
Low V
differential, <0.5V
Low input voltage, <5V
Low input voltage, <4.5V
x f x VL
x t
IN
Low-Voltage, Precision Step-Down
) + 20ns]
Controller for Portable CPU Power
D
SENSE
x I
term assumes identi-
CONDITION
IN
IN
IN
x f
LOAD
-V
-V
-V
OUT
OUT
OUT
is the current-
x f x 3/2 x
DS(ON)
GATE
g
is
is
is
Limited inductor-current
slew rate per cycle.
Maximum duty-cycle limits
exceeded.
Normal function of internal
low-dropout circuitry.
VL linear regulator is going
into dropout and isn’t provid-
ing good gate-drive levels.
VL output is so low that it
hits the VL UVLO threshold.
ROOT CAUSE
dissipated in the MOSFET body diode if no external
Schottky diode is used.
where I
the Input Capacitor Value section.
Under light loads, the PWM operates in discontinuous
mode, where the inductor current discharges to zero at
some point during the switching cycle. This makes the
inductor current’s AC component high compared to the
load current, which increases core losses and I
es in the output filter capacitors. For best light-load effi-
ciency, use MOSFETs with moderate gate-charge
levels and use ferrite, MPP, or other low-loss core mate-
rial. Avoid powdered-iron cores; even Kool-Mu
(aluminum alloy) is not as good as ferrite.
Good PC board layout is required in order to achieve
specified noise, efficiency, and stable performance.
The PC board layout artist must be given explicit
instructions, preferably a pencil sketch showing the
placement of power-switching components and high-
current routing. See the PC board layout in the
MAX1636 evaluation kit manual for examples. A ground
plane is essential for optimum performance. In most
applications, the circuit will be located on a multi-layer
board, and full use of the four or more copper layers
is recommended. Use the top layer for high-current
P(cap) = input capacitor ESR loss = I
RMS
Light-Load Efficiency Considerations
is the input ripple current as calculated in
PC Board Layout Considerations
Increase bulk output capacitance
per formula (see Low-Voltage
Operation section). Reduce inductor
value.
Reduce operation to 200kHz.
Reduce MOSFET on-resistance and
coil DCR.
Increase the minimum input voltage
or ignore.
Use a small 20mA Schottky diode
for boost diode. Supply VL from an
external source.
Supply VL from an external source
other than V
+5V supply.
SOLUTION
IN
, such as the system
RMS 2
x R
2
R loss-
ESR
21