MAX796 Maxim, MAX796 Datasheet - Page 23

• Manufacturer Part Number: MAX796
• Description: Step-Down Controllers with Synchronous Rectifier for CPU Power
• Manufacturer: Maxim

Low input voltages and low input-output differential volt-
ages each require some extra care in the design. Low
absolute input voltages can cause the VL linear regulator
to enter dropout, and eventually shut itself off. Low input
voltages relative to the output (low V
can cause bad load regulation in multi-output flyback
applications. See the design equations in the Transformer
Design section. Finally, low V
cause the output voltage to sag when the load current
changes abruptly. The amplitude of the sag is a function
of inductor value and maximum duty factor (an Electrical
Characteristics parameter, 93% guaranteed over temper-
ature at f = 150kHz) as follows:
The cure for low-voltage sag is to increase the value of
the output capacitor. For example, at V
= 5V, L = 10µH, f = 150kHz, a total capacitance of
660µF will prevent excessive sag. Note that only the
capacitance requirement is increased and the ESR
requirements don’t change. Therefore, the added
capacitance can be supplied by a low-cost bulk
capacitor in parallel with the normal low-ESR capacitor.
Table 4. Low-Voltage Troubleshooting
Sag or droop in V
under step load change
Dropout voltage is too
high (V
V
Unstable—jitters between
two distinct duty factors
Secondary output won’t
support a load
High supply current,
poor efficiency
Won’t start under load or
quits before battery is
completely dead
____________Low-Voltage Operation
IN
decreases)
V
OUT
SYMPTOM
SAG
follows V
= ———————————————
2 x C
OUT
______________________________________________________________________________________
IN
F
as
x (V
(I
Low V
<1.5V
Low V
<1V
Low V
<1V
Low V
V
(MAX796/MAX799 only)
Low input voltage, <5V
Low input voltage, <4.5V
Synchronous Rectifier for CPU Power
IN(MIN)
IN
STEP
IN
< 1.3 x V
-V
IN
IN
IN
IN
CONDITION
)
OUT
2
-V
-V
-V
-V
x L
x D
OUT
OUT
OUT
OUT
differentials can also
IN
OUT
MAX
differential,
differential,
differential,
differential,
-V
IN
(main)
OUT
= 5.5V, V
- V
OUT
differential)
)
Step-Down Controllers with
Limited inductor-current slew
rate per cycle.
Maximum duty-cycle limits
exceeded.
Inherent limitation of fixed-fre-
quency current-mode SMPS
slope compensation.
Not enough duty cycle left to
initiate forward-mode operation.
Small AC current in primary can’t
store energy for flyback operation.
VL linear regulator is going into
dropout and isn’t providing
good gate-drive levels.
VL output is so low that it hits the
VL UVLO threshold at 4.2V max.
OUT
ROOT CAUSE
The major efficiency loss mechanisms under loads are,
in the usual order of importance:
Inductor-core losses are fairly low at heavy loads
because the inductor’s AC current component is small.
Therefore, they aren’t accounted for in this analysis.
Ferrite cores are preferred, especially at 300kHz, but
powdered cores such as Kool-mu can work well.
where R
the MOSFET on-resistance, and R
__________Applications Information
Efficiency = P
P
P(I
TOTAL
of the IC
P(I
P(gate), gate-charge losses
P(diode), diode-conduction losses
P(tran), transition losses
P(cap), capacitor ESR losses
P(IC), losses due to the operating supply current
Heavy-Load Efficiency Considerations
2
2
R) = (I
DC
R), I
= P(I
is the DC resistance of the coil, R
2
P(cap) + P(IC)
= P
R losses
LOAD
2
OUT
OUT
R) + P(gate) + P(diode) + P(tran) +
Increase bulk output capacitance per
formula above. Reduce inductor value.
Reduce f to 150kHz. Reduce MOSFET
on-resistance and coil DCR.
Reduce L value. Tolerate the remaining
jitter (extra output capacitance helps
somewhat).
Reduce f to 150kHz. Reduce secondary
impedances—use Schottky if possible.
Stack secondary winding on main output.
Use a small 20mA Schottky diode for
boost diode D2. Supply VL from an
external source.
Supply VL from an external source other
than V
)
2
/ P
/ (P
x (R
BATT
IN
OUT
DC
x 100%
, such as the system 5V supply.
+ P
+ R
SOLUTION
TOTAL
DS(ON)
SENSE
) x 100%
+ R
is the current-
SENSE
DS(ON)
)
23
is