MAX1565 Datasheet, PDF (19/26 Pages) Maxim Integrated Products – Small, High-Efficiency, Five-Channel Digital Still Camera Power Supply

Small, High-Efficiency, Five-Channel
Digital Still Camera Power Supply
Choose f_C = 20kHz. Calculate C_C:
C_C = (V_FB/V_OUT)(R_LOAD/R_CS)(gm/2π f_C)(1 - D)
= (1.25/3.35)(6.7/0.3) x (135µS/(6.28 x 20kHz)
(2/3.35) = 5.35nF
Choose 6.8nF. Now select R_C such that 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 tran-
sient gain. Since the current-sense transresistance is
0.3V/A, the value of R_C that allows the required load
step swing:
R_C = 0.3 I_IND(PK)/6.75µA
In a step-up DC-to-DC converter, if L_IDEAL is used, out-
put current relates to inductor current by:
I_IND(PK) = 1.25 I_OUT/(1 - D) = 1.25 I_OUT V_OUT/V_IN
Thus, for a 400mA output load step with V_IN = 2V and
V_OUT = 3.35V:
R_C = [1.25(0.3 x 0.4 x 3.35)/2)]/6.75µA = 37kΩ
Note that the inductor does not limit the response in this
case since it can ramp at 2V/3.3µH, or 606mA/µs. The
output filter capacitor is then chosen so that the C_OUT
R_LOAD pole cancels the R_C C_C zero:
C_OUT R_LOAD = R_CC_C
For example:
C_OUT = 37kΩ x 6.8nF/6.7 = 37.5µF
Since a reasonable value for C_OUT is 47µF rather than
37.5, choose 47µF and rescale RC:
R_C = 47µF x 6.7/6.8nF = 46.3kΩ
which provides a slightly higher transient gain and con-
sequently less transient droop than previously selected.
If the output filter capacitor has significant ESR, a zero
occurs at:
Z_ESR = 1/(2π C_OUT 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 COMPSU to GND:
C_P = C_OUT 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
compensation RC network. The MAX1565 step-down
converter provides best efficiency with continuous
inductor current. A reasonable inductor value (LIDEAL)
can be derived from:
L_IDEAL = 2 (V_IN) D (1 - D)/(I_OUT f_OSC)
which sets the peak-to-peak inductor current at 1/2 the
DC inductor current. D is the duty cycle:
D = V_OUT/V_IN
Given L_IDEAL, the peak-to-peak inductor current varia-
tion is 0.5 I_OUT. The absolute peak inductor current is
1.25 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:
1) Transconductance (from FBSD to COMPSD), gm_EA
(135µS)
2) Step-down slope compensation pole, P_SLOPE =
V_IN / (πL)
3) Current-sense amplifier transresistance, R_CS,
(0.6V/A)
4) Feedback regulation voltage, V_FB (1.25V)
5) Step-down output voltage, V_SD, in V
6) Output load equivalent resistance, R_LOAD,
in Ω = V_OUTSD/I_LOAD
The key steps for step-down compensation are:
1) Set the compensation RC zero to cancel the R_LOAD
C_OUT pole.
2) Set the loop crossover below the lower of 1/5 the
slope compensation pole, or 1/5 the switching fre-
quency.
If we assume V_IN = 3.35V, V_OUT = 1.5V, and I_OUT =
350mA, then R_LOAD = 4.3Ω.
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