Rainbow Electronics MAX1636 - Manual (Page 21)

where R DC is the DC resistance of the coil, R DS(ON) is
the MOSFET on-resistance, and R
SENSE
is the current-
sense resistor value. The R DS(ON) term assumes identi-
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.
PD(tran) = transition loss = V IN x I LOAD x f x 3/2 x
[(V
IN
C
RSS
/ I
GATE
) + 20ns]
where C RSS is the reverse transfer capacitance of the
high-side MOSFET (a data-sheet parameter), I GATE is
the DH gate-driver peak output current (1.5A typ), and
20ns is the rise/fall time of the DH driver (20ns typ).
P(gate) = Q g x f x VL
where VL is the internal logic-supply voltage (+5V), and
Q
g is the sum of the gate-charge values for low-side
and high-side switches. For matched MOSFETs, Q g is
twice the data-sheet value of an individual MOSFET. If
V
OUT
is set to less than 4.5V, replace VL in this equa-
tion with V BATT . In this case, efficiency can be
improved by connecting VL to an efficient 5V source,
such as the system +5V supply.
P(diode) = diode conduction losses =
I LOAD x V FWD x t D x f
where t D is the diode-conduction time (120ns typ), and
V
FWD
is the forward voltage of the diode. This power is
dissipated in the MOSFET body diode if no external
Schottky diode is used.
P(cap) = input capacitor ESR loss = I RMS2 x R ESR
where I RMS is the input ripple current as calculated in
the Input Capacitor Value section.
Light-Load Efficiency Considerations
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
2 R loss-
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.
PC Board Layout Considerations
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
MAX1636
Low-Voltage, Precision Step-Down
Controller for Portable CPU Power
______________________________________________________________________________________ 21
CONDITION
Low V IN -V OUT
differential, <1V
Maximum duty-cycle limits
exceeded.
Low V IN -V OUT
differential, <1.5V
Dropout voltage is too high
(V
OUT follows V IN as V IN
decreases)
Reduce operation to 200kHz.
Reduce MOSFET on-resistance and
coil DCR.
SYMPTOM
Low V IN -V OUT
differential, <0.5V
ROOT CAUSE
Normal function of internal
low-dropout circuitry.
Unstable—jitters between
different duty factors and
frequencies
Limited inductor-current
slew rate per cycle.
Increase the minimum input voltage
or ignore.
Low input voltage, <4.5V
VL output is so low that it
hits the VL UVLO threshold.
Low input voltage, <5V
Won’t start under load or
quits before battery is
completely dead Supply VL from an external source
other than V
IN , such as the system
+5V supply.
VL linear regulator is going
into dropout and isn’t provid-
ing good gate-drive levels.
SOLUTION
Poor efficiency
Use a small 20mA Schottky diode
for boost diode. Supply VL from an
external source.
Sag or droop in V OUT under
step-load change
Increase bulk output capacitance
per formula (see Low-Voltage
Operation section). Reduce inductor
value.
Table 6. Low-Voltage Troubleshooting Chart