Rainbow Electronics MAX1639 - Manual (Page 10)

Overvoltage Protection
When the output exceeds the set voltage, the synchro-
nous rectifier output (DL) is driven high (and DH is dri-
ven low). This causes the inductor to quickly dissipate
any stored energy and force the fault current to flow to
ground. Current is limited by the source impedance
and parasitic resistance of the current path, so a fuse is
required in series with the +5V input to protect against
low-impedance faults, such as a shorted high-side
MOSFET. Otherwise, the low-side MOSFET will eventu-
ally fail. DL will go low if the input voltage drops below
the undervoltage lockout point.
Internal Soft-Start
Soft-start allows a gradual increase of the internal cur-
rent limit at start-up to reduce input surge currents. An
internal DAC raises the current-limit threshold from 0V
to 100mV in four steps (25mV, 50mV, 75mV, and
100mV) over the span of 1536 oscillator cycles.
__________________Design Procedure
Setting the Output Voltage
Set the output voltage by connecting R7 and R8 (Figure 6)
to the FB pin from the output to AGND. R7 is given by the
following equation:
where V FB = 1.1V. Since the input bias current at FB
has a maximum value of ±0.1µA, values up to 10k
Ω
can be used for R8 with no significant accuracy loss.
Values under 1k
Ω
are recommended to improve noise
immunity. Place R7 and R8 very close to the MAX1639,
within 0.2in (5mm) of the FB pin.
Feed-Forward Compensation
An optional compensation capacitor (C8), typically
220pF, may be needed across the upper feedback
resistor to counter the effects of stray capacitance on the
FB pin, and to help ensure stable operation when high-
value feedback resistors are used (Figure 6). Empirically
adjust the feed-forward capacitor as needed.
Specifying the Inductor
Three key inductor parameters must be specified:
inductance value (L), peak current (I
PEAK ), and DC
resistance (R DC ). The following equation includes a
constant LIR, which is the ratio of inductor peak-to-
peak AC current to DC load current. Typically LIR can
be between 0.1 to 0.5. A higher LIR value allows for
smaller inductors and better transient response, but
results in higher losses and output ripple. A good com-
promise between size and loss is a 30% ripple current
to load current ratio (LIR = 0.30), which corresponds to
a peak inductor current 1.15 times higher than the DC
load current.
R R x
V
V
OUT
FB
7 8 1
= −

 

MAX1639
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power
10 ______________________________________________________________________________________
C10
4.7nF
R1
C9
4.7nF
R9
39 Ω
R5
10 Ω
R10
39 Ω
CSH
V CC
CSL
MAX1639
N1
C5
0.1 µ F
C6
10 µ F
C1
V
IN
Figure 5. High-Side Current Sense
R8
PLACE VERY CLOSE
TO MAX1639
R7
FB
AGND
V OUT
C8 (OPTIONAL)
LOAD MAX1639
Figure 6. Output Selection