Rainbow Electronics MAX1625 User Manual
Page 20
MAX1624/MAX1625
High-Speed Step-Down Controllers with
Synchronous Rectification for CPU Power
20
______________________________________________________________________________________
Calculating the Loop Gain (Optional)
The loop gain is an important parameter in alternative
compensation schemes:
where A
E
is the error-comparator relative gain, and
A
I
= 10 is the integrator gain. A
E
is 4 for the MAX1625,
but it is 2, 4, or 8 for LG pin settings of V
CC
, REF, or
AGND, respectively, for the MAX1624.
Feed-Forward Compensation (MAX1625)
An optional compensation capacitor, 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 9). Empirically adjust
the feed-forward capacitor as needed.
Choosing the MOSFET Switches
The two high-current N-channel MOSFETs must be
logic-level types with guaranteed on-resistance specifi-
cations at V
GS
= 4.5V. Lower gate-threshold specs are
better (i.e., 2V max rather than 3V max). Gate charge
should be less than 100nC to minimize switching losses
and reduce power dissipation.
I
2
R losses are the greatest heat contributor to MOSFET
power dissipation and are distributed between the high-
and low-side MOSFETs according to duty factor, as follows:
Switching losses affect the upper MOSFET only, and are
insignificant at 5V input voltages. Gate-charge losses are
dissipated in the IC, and do not heat the MOSFETs.
Ensure that both MOSFETs are at a safe junction temper-
ature by calculating the temperature rise according to
package thermal-resistance specifications. The high-side
MOSFET’s worst-case dissipation occurs at the maximum
output voltage and minimum input voltage. For the low-
side MOSFET, the worst case is at the maximum input
voltage when the output is short-circuited (consider the
duty factor to be 100%).
Selecting the Rectifier Diode
The rectifier diode D1 is a clamp that catches the nega-
tive inductor swing during the 30ns typical dead time
between turning off the high-side MOSFET and turning on
the low-side MOSFET synchronous rectifier. D1 must be a
Schottky diode, to prevent the MOSFET body diode from
conducting. It is acceptable to omit D1 and let the body
diode clamp the negative inductor swing, but efficiency
will drop 1% or 2% as a result. Use a 1N5819 diode for
loads up to 3A, or a 1N5822 for loads up to 10A.
Adding the BST Supply Diode
and Capacitor
A signal diode, such as a 1N4148, works well for D2 in
most applications, although a low-leakage Schottky diode
provides slightly improved efficiency. Do not use large
power diodes, such as the 1N4001 or 1N5817. Exercise
caution in the selection of Schottky diodes, since some
types exhibit high reverse leakage at high operating tem-
peratures. Bypass BST to LX using a 0.1µF capacitor.
Selecting the Input Capacitors
Place a 0.1µF ceramic capacitor and 4.7µF capacitor
between V
CC
and AGND, as well as between V
DD
and
PGND, within 0.2 in. (5mm) of the V
CC
and V
DD
pins.
Select low-ESR input filter capacitors with a ripple-
current rating exceeding the RMS input ripple current,
connecting several capacitors in parallel if necessary.
RMS input ripple current is determined by the input
voltage and load current, with the worst-possible case
occurring at V
IN
= 2 x V
OUT
:
I
I
V
V
V
V
I
I
when V
V
RMS
LOAD MAX
OUT
IN
OUT
IN
RMS
OUT
IN
OUT
(
)
/
(
)
=
−
=
=
2
2
P
low side
I
x R
x
V
V
P
low side shorted
I
x R
where I
mV R
D
LOAD
DS ON
OUT
IN
D
LIMIT
DS ON
LIMIT
SENSE
(
)
(
,
)
/
.
(
)
(
)
=
−
=
=
2
1
115
2
P
high side
I
x R
x
V
V
D
LOAD
DS ON
OUT
IN
(
)
(
)
=
2
Loop Gain dB
Log
A
V
V
x
R
R
x A
Log
A
V
mV
x
E
REF
OUT
LOAD
CS
I
E
REF
(
)
=
=
20
20
85
10
R3
R2
OPTIONAL FEED-
FORWARD CAPACITOR
FB
AGND
OUTPUT
MAX1625
Figure 9. MAX1625 Optional Feed-Forward Compensation
Capacitors