Rainbow Electronics MAX1625 User Manual
Page 17
MAX1624/MAX1625
High-Speed Step-Down Controllers with
Synchronous Rectification for CPU Power
______________________________________________________________________________________
17
MAX1625
Set the output voltage by connecting R2 and R3 (Fig-
ure 7) to the FB pin from the output to AGND. R2 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 100k
Ω
can be used for R3 with no significant accuracy loss.
Values under 1k
Ω
are recommended to improve noise
immunity and minimize parasitic capacitance at the FB
node. Place R2 and R3 very close to the MAX1625,
within 0.2 in. (5mm) of the FB pin.
Selecting the Oscillator Frequency
Set the switching frequency between 100kHz and
1MHz by connecting a resistor from FREQ to AGND.
Select the resistor according to the following equation:
Low-frequency operation reduces controller IC quies-
cent current and improves efficiency. High-frequency
operation reduces cost and PC board area by allowing
the use of smaller inductors and fewer and smaller out-
put capacitors. Inductor energy-storage requirements
and output capacitor requirements at 1MHz are one-
third those at 300kHz.
Choosing the
Error-Amplifier Gain (MAX1624)
Set the error-amplifier gain to match the voltage-preci-
sion requirements of the CPU used. The MAX1624’s
loop-gain control input (LG) allows trade-offs in DC/AC
voltage accuracy versus output filter capacitor require-
ments. AC load regulation can be set to 0.5%, 1%,
or 2% by connecting LG as shown in Table 5. The
MAX1625’s default AC regulation is 1%.
DC load regulation is typically 10 times better than AC
load regulation, and is determined by the gain set by
the LG pin.
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. A higher LIR
value allows for smaller inductors and better transient
response, but results in higher losses and output ripple.
R
x
f
OSC
4
2
10
=
10
R
R
x
V
V
OUT
FB
2
3
1
=
−
AC LOAD-
REGULATION
ERROR
(%)
1
LG
CONNECTED
TO
DC LOAD-
REGULATION
ERROR
(%)
REF
0.1
GND
0.05
V
CC
0.2
0.5
2
TYPICAL
A
E
(V
GAIN
/
I
GAIN
)
8
2
4
Table 4. Output Voltage-Adjustment
Settings
Table 5. LG Pin Adjustment Settings
Non-Intel-
compatible
DAC codes
No CPU (off)
1
1
1
1
0
1.1
0
1
1
1
0
1.1
—
—
—
1
0
1.1
1
0
0
1
0
1.1
0
0
0
1
0
1.2
1
1
1
0
0
1.3
0
1
1
0
0
1.4
1
0
1
0
0
1.5
0
0
1
0
0
1.6
1
1
0
0
0
1.7
0
1
0
0
0
1.8
1
0
0
0
0
1.9
0
0
0
0
0
Intel-compatible
DAC codes
No CPU (off)
1
1
1
1
1
2.1
0
1
1
1
1
2.2
1
0
1
1
1
COMPATIBILITY
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
3.5
3.1
3.3
3.2
3.4
OUTPUT
VOLTAGE (V)
0
1
1
0
0
1
1
0
0
0
1
1
0
D1
1
1
1
1
1
0
0
0
0
0
0
0
0
1
1
1
0
1
0
0
1
1
0
1
0
0
1
1
1
1
0
1
1
1
1
1
0
0
1
0
1
1
0
0
1
1
0
1
1
0
1
D0
D2
D4
0
D3