MAX1565 Datasheet, PDF(20/26 Page) Maxim Integrated Products – Small, High-Efficiency, Five-Channel Digital Still Camera Power Supply

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MAX1565 Datasheet, PDF (20/26 Pages) Maxim Integrated Products – Small, High-Efficiency, Five-Channel Digital Still Camera Power Supply

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Small, High-Efficiency, Five-Channel

Digital Still Camera Power Supply

If we select L = 4.7ÂµH and fOSC = 440kHz, PSLOPE =

VIN/(ÏL) = 214kHz, so choose fC = 40kHz and calculate CC:

CC = (VFB/VOUT)(RLOAD/RCS)(gm/2Ï fC)

= (1.25/1.5)(4.3/0.6) x (135ÂµS/(6.28 x 40kHz)

= 3.2nF

Choose 3.3nF. Now select RC 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 135ÂµS, or 6.75ÂµA across RC to provide tran-

sient gain. Since the current-sense transresistance is

0.6V/A, the value of RC that allows the required load

step swing:

RC = 0.6 IIND(PK)/6.75ÂµA

In a step-down DC-to-DC converter, if LIDEAL is used,

output current relates to inductor current by:

IIND(PK) = 1.25 IOUT

Thus, for a 250mA output load step with VIN = 3.35V

and VOUT = 1.5V:

RC = (1.25 x 0.6 x 0.25)/6.75ÂµA = 27.8kâ¦

Choose 27kâ¦. Note that the inductor does not limit the

response in this case since it can ramp at (VIN -

VOUT)/4.7ÂµH, or (3.35 - 1.5)/4.7ÂµH = 394mA/Âµs.

The output filter capacitor is then chosen so that the

COUT RLOAD pole cancels the RC CC zero:

COUTRLOAD = RCCC

For example:

COUT = 27kâ¦ x 3.3nF/4.3 = 20.7ÂµF

Choose 22ÂµF. If the output filter capacitor has signifi-

cant ESR, a zero occurs at:

ZESR = 1/(2Ï COUTRESR)

If ZESR > fC, it can be ignored, as is typically the case

with ceramic output capacitors. If ZESR is less than fC,

it should be cancelled with a pole set by capacitor CP

connected from COMPSD to GND:

CP = COUTRESR/RC

If CP is calculated to be < 10pF, it can be omitted.

Auxiliary Controller Component Selection

External MOSFET

All MAX1565 auxiliary controllers drive external logic-

level N-channel MOSFETs. Significant MOSFET selec-

tion parameters are:

1) On-resistance (RDS(ON))

2) Maximum drain-to-source voltage (VDS(MAX))

3) Total gate charge (QG)

4) Reverse transfer capacitance (CRSS)

DL_ swings between OUTSU and GND. Use a MOSFET

with on-resistance specified at or below the main output

voltage. The gate charge, QG, includes all capacitance

associated with charging the gate and helps to predict

MOSFET transition time between on and off states.

MOSFET power dissipation is a combination of

on-resistance and transition losses. The on-resistance

loss is:

PRDSON = D IL2 RDS(ON)

where D is the duty cycle, IL is the average inductor

current, and RDS(ON) is MOSFET on-resistance. The

transition loss is approximately:

PTRANS = (VOUT IL fOSC tT)/3

where VOUT is the output voltage, IL is the average

inductor current, fOSC is the switching frequency, and

tT is the transition time. The transition time is approxi-

mately QG/IG, where QG is the total gate charge, and

IG is the gate drive current (typically 0.5A). The total

power dissipation in the MOSFET is:

PMOSFET = PRDSON + PTRANS

Diode

For most auxiliary applications, a Schottky diode rectifies

the output voltage. The Schottky diodeâs low forward volt-

age and fast recovery time provide the best performance

in most applications. Silicon signal diodes (such as

1N4148) are sometimes adequate in low-current

(<10mA) high-voltage (>10V) output circuits where the

output voltage is large compared to the diode forward

voltage.

Auxiliary Compensation

The auxiliary controllers employ voltage-mode control

to regulate their output voltage. Optimum compensa-

tion somewhat depends on whether the design uses

continuous or discontinuous inductor current.

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