MAX1566-MAX1567 Datasheet, PDF(30/35 Page) Maxim Integrated Products – Six-Channel, High-Efficiency, Digital Camera Power Supplies

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MAX1566-MAX1567 Datasheet, PDF (30/35 Pages) Maxim Integrated Products – Six-Channel, High-Efficiency, Digital Camera Power Supplies

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Six-Channel, High-Efficiency, Digital

Camera Power Supplies

For the example:

COUT = 27kâ¦ x 6.8nF / 5.14â¦ = 35.7ÂµF

Since ceramic capacitors are common in either 22ÂµF or

47ÂµF values, 22ÂµF is within a factor of two of the ideal value

and still provides adequate phase margin for stability. If

the output filter capacitor has significant ESR, a zero

occurs at the following:

ZESR = 1 / (2Ï x COUT x RESR)

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

with ceramic output capacitors. If ZESR < fC, it should

be cancelled with a pole set by capacitor CP connect-

ed from CC to GND:

CP = COUT x RESR / RC

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

AUX Controller Component Selection

External MOSFET

All MAX1566/MAX1567 AUX controllers drive external

logic-level MOSFETs. Significant MOSFET selection

parameters are as follows:

â¢ On-resistance (RDS(ON))

â¢ Maximum drain-to-source voltage (VDS(MAX))

â¢ Total gate charge (QG)

â¢ Reverse transfer capacitance (CRSS)

On the MAX1566, all AUX drivers are designed for N-

channel MOSFETs. On the MAX1567, AUX2 is a DC-to-

DC inverter, so DL2 is designed to drive a P-channel

MOSFET. In both devices, the driver outputs DL1 and

DL3 swing between PVSU and GND. MOSFET driver

DL2 swings between INDL2 and GND.

Use a MOSFET with on-resistance specified with gate

drive at or below the main output voltage. The gate

charge, QG, includes all capacitance associated with

charging the gate and helps to predict MOSFET transi-

tion time between on and off states. MOSFET power

dissipation is a combination of on-resistance and tran-

sition losses. The on-resistance loss is as follows:

PRDSON = D x IL2 x 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 as follows:

PTRANS = (VOUT x IL x fOSC x 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 (0.5A typ).

The total power dissipation in the MOSFET is as follows:

PMOSFET = PRDSON + PTRANS

Diode

For most AUX applications, a Schottky diode rectifies

the output voltage. Schottky low forward voltage 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.

AUX Compensation

The auxiliary controllers employ voltage-mode control

to regulate their output voltage. Optimum compensa-

tion depends on whether the design uses continuous or

discontinuous inductor current.

AUX Step-Up, Discontinuous Inductor Current

When the inductor current falls to zero on each switch-

ing cycle, it is described as discontinuous. The inductor

is not utilized as efficiently as with continuous current,

but in light-load applications this often has little negative

impact since the coil losses may already be low com-

pared to other losses. A benefit of discontinuous induc-

tor current is more flexible loop compensation, and no

maximum duty-cycle restriction on boost ratio.

To ensure discontinuous operation, the inductor must

have a sufficiently low inductance to fully discharge on

each cycle. This occurs when:

L < [VIN2 (VOUT - VIN) / VOUT3] [RLOAD / (2fOSC)]

A discontinuous current boost has a single pole at the

following:

fP = (2VOUT - VIN) / (2Ï x RLOAD x COUT x VOUT)

Choose the integrator cap so the unity-gain crossover,

fC, occurs at fOSC / 10 or lower. Note that for many AUX

circuits, such as those powering motors, LEDs, or other

loads that do not require fast transient response, it is

often acceptable to overcompensate by setting fC at

fOSC / 20 or lower.

CC is then determined by the following:

CC = [2VOUT x VIN / ((2VOUT - VIN) x VRAMP)] [VOUT /

(K(VOUT - VIN))]1/2 [(VFB / VOUT)(gM / (2Ï x fC))]

where:

K = 2L x fOSC / RLOAD

and VRAMP is the internal slope-compensation voltage

ramp of 1.25V.

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