MAX1638 Datasheet, PDF (13/16 Pages) Maxim Integrated Products – High-Speed Step-Down Controller with Synchronous Rectification for CPU Power

High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power
Table 2. Output Voltage Adjustment
Settings
D4 D3 D2 D1 D0
00 0 00
00 0 01
00 0 10
00 0 11
00 1 00
00 1 01
00 1 10
00 1 11
01 0 00
01 0 01
01 0 10
01 0 11
01 1 00
01 1 01
01 1 10
01 1 11
10 0 00
10 0 01
10 0 10
10 0 11
10 1 00
10 1 01
10 1 10
10 1 11
11 0 00
11 0 01
11 0 10
11 0 11
11 1 00
11 1 01
11 1 10
11 1 11
OUTPUT
VOLTAGE
(V)
2.050
2.000
1.950
1.900
1.850
1.800
1.750
1.700
1.650
1.600
1.550
1.500
1.450
1.400
1.350
1.300
3.500
3.400
3.300
3.200
3.100
3.000
2.900
2.800
2.700
2.600
2.500
2.400
2.300
2.200
2.100
N/A
COMPATIBILITY
Intel-compatible
DAC codes
Continuation of
50mV increment
to 1.3V
Intel-compatible
DAC codes
Shutdown
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
Table 3. LG Pin Adjustment Settings
LG
CONNECTED
TO:
AC LOAD-
REGULATION
ERROR
(%)
DC LOAD-
REGULATION
ERROR
(%)
V_CC
2
0.2
REF
1
0.1
GND
0.5
0.05
TYPICAL
A_E
(V_GAIN/
I_GAIN)
2
4
8
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.
( ) ^VOUT ^VIN(MAX) ^{− V}OUT
L=
V_IN(MAX) x f_OSC x I_OUT x LIR
where f is the switching frequency, between 300kHz and
1MHz; I_OUT is the maximum DC load current; and LIR is
the ratio of AC to DC inductor current (typically 0.3). The
exact inductor value is not critical and can be adjusted to
make trade-offs among size, transient response, cost,
and efficiency. Although lower inductor values minimize
size and cost, they also reduce efficiency due to higher
peak currents. In general, higher inductor values
increase efficiency, but at some point resistive losses
due to extra turns of wire exceed the benefit gained from
lower AC current levels. Load-transient response can be
adversely affected by high inductor values, especially at
low (V_IN - V_OUT) differentials.
The peak inductor current at full load is 1.15 x I_OUT if the
previous equation is used; otherwise, the peak current
can be calculated using the following equation:
( ) ^VOUT ^VIN(MAX) ^{− V}OUT
I_PEAK = I_OUT + _{2fOSC x L x VIN(MAX)}
The inductor’s DC resistance is a key parameter for effi-
cient performance, and should be less than the current-
sense resistor value.
Calculating the Current-Sense
Resistor Value
Calculate the current-sense resistor value according to
the worst-case minimum current-limit threshold voltage
(from the Electrical Characteristics) and the peak
inductor current required to service the maximum load.
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