Rainbow Electronics MAX1585 - Manual (Page 21)

If Z ESR > f C , it can be ignored, as is typically the case
with ceramic output capacitors. If Z
ESR
is less than f
C
,
it should be cancelled with a pole set by capacitor C P
connected from CCSU to GND:
C P = C OUT x R ESR / R C
If C P is calculated to be <10pF, it can be omitted.
Step-Down Component Selection
Step-Down Inductor
The external components required for the step-down
are an inductor, input and output filter capacitors, and
a compensation RC network.
The MAX1585/1585 step-down converter provides best
efficiency with continuous inductor current. A reason-
able inductor value (L
IDEAL
) can be derived from the
following:
L
IDEAL
= [2(V
IN
) x D(1 - D)] / I
OUT
x f
OSC
which sets the peak-to-peak inductor current at half the
DC inductor current. D is the duty cycle:
D = V OUT / V IN
Given L IDEAL , the peak-to-peak inductor current is 0.5 x
I OUT . The absolute peak inductor current is 1.25 x I OUT .
Inductance values smaller than L IDEAL can be used to
reduce inductor size; however, if much smaller values
are used, inductor current rises and a larger output
capacitance may be required to suppress output ripple.
Larger values than L
IDEAL can be used to obtain higher
output current, but with typically larger inductor size.
Step-Down Compensation
The relevant characteristics for step-down compensa-
tion are as follows:
• Transconductance (from FBSD to CCSD), g MEA
(135µS)
• Current-sense amplifier transresistance, R CS (0.6V/A)
• Feedback regulation voltage, V FB (1.25V)
• Step-down output voltage, V
SD
, in V
• Output load equivalent resistance, R LOAD , in
Ω = V
SD
/ I
LOAD
The key steps for step-down compensation are as fol-
lows:
1) Set the compensation RC zero to cancel the R LOAD
C OUT pole.
2) Set the loop crossover below 1/10 the switching fre-
quency.
If we assume V
IN
= 3.5V, V
OUT
= 1.5V, and I
OUT
=
250mA, then R LOAD = 6 Ω.
If we select f
OSC
= 500kHz and L = 22µH,
choose f C = 24kHz and calculate C C :
C C = (V FB / V OUT )(R LOAD / R CS )(g M / 2
π x f
C )
= (1.25 / 1.5)(6 / 0.6) x (135µS / (6.28 x 40kHz))
= 4.5nF
Choose 4.7nF.
Now select R
C
so transient-droop requirements are met.
For example, if 4% transient droop is allowed, the input
to the error amplifier moves 0.04 x 1.25V, or 50mV. The
error-amp output drives 50mV x 135µS, or 6.75µA across
R
C to provide transient gain. Since the current-sense
transresistance is 0.6V/A, the value of R C that allows the
required load step swing is as follows:
R C = 0.6 x I IND(PK) / 6.75µA
In a step-down DC-DC converter, If L IDEAL is used, out-
put current relates to inductor current by the following:
I IND(PK) = 1.25 x I OUT
So for a 250mA output load step with V IN = 3.5V and
V OUT = 1.5V:
R C = (1.25 x 0.6 x 0.25) / 6.75µA = 27.8k Ω
Choose 27k Ω.
The inductor does somewhat limit the response in this
case since it ramps at (V
IN
- V
OUT
) / 22µH, or (3.5 - 1.5)
/ 22µH = 90mA/µs.
The output filter capacitor is then chosen so the C
OUT
R LOAD pole cancels the R C C C zero:
C
OUT
x R
LOAD
= R
C
x C
C
For the example:
C OUT = 27k
Ω x 4.7nF / 6Ω = 21µF
Choose 22µF or greater.
If the output filter capacitor has significant ESR, a zero
occurs at:
Z
ESR
= 1 / (2 π x C
OUT
x R
ESR
)
If Z ESR > f C , it can be ignored, as is typically the case
with ceramic output capacitors. If Z ESR is less than f C ,
it should be cancelled with a pole set by capacitor C P
connected from CCSD to GND:
C P = C OUT x R ESR / R C
If C P is calculated to be <10pF, it can be omitted.
AUX Controller Component Selection
External MOSFET
MAX1584/MAX1585 AUX1(step-up) controllers drive
external logic-level N-channel MOSFETs. AUX3 (step-
down) controllers drive P-channel MOSFETs. AUX2
(step-up) on the MAX1584 drives an N channel, while
AUX2 (inverting) on the MAX1585 drives a P channel.
MAX1584/MAX1585
5-Channel Slim DSC Power Supplies
______________________________________________________________________________________ 21