MAX1584 Datasheet, PDF (21/29 Pages) Maxim Integrated Products – 5-Channel Slim DSC Power Supplies

5-Channel Slim DSC Power Supplies
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 = 6Ω.
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 / 2π 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 = 27.8kΩ
Choose 27kΩ.
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 = 27kΩ x 4.7nF / 6Ω = 21µF
Choose 22µF or greater.
If the output filter capacitor has significant ESR, a zero
occurs at:
Z_ESR = 1 / (2π 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.
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