MAX1652EEE datasheet(23/28 Pages) MAXIM

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High-Efficiency, PWM, Step-Down

DC-DC Controllers in 16-Pin QSOP

______________________________________________________________________________________

Boost-Supply Diode D2

A 10mA to 100mA Schottky diode or signal diode such

as a 1N4148 works well for D2 in most applications. If

the input voltage can go below 6V, use a Schottky

diode for slightly improved efficiency and dropout char-

acteristics. Don’t use large power diodes such as

1N5817 or 1N4001, since high junction capacitance

can cause VL to be pumped up to excessive voltages.

Rectifier Diode D3

(Transformer Secondary Diode)

The secondary diode in coupled-inductor applications

must withstand high flyback voltages greater than 60V,

which usually rules out most Schottky rectifiers.

Common silicon rectifiers such as the 1N4001 are also

prohibited, as they are far too slow. This often makes

fast silicon rectifiers such as the MURS120 the only

choice. The flyback voltage across the rectifier is relat-

ed to the VIN-VOUT difference according to the trans-

former turns ratio:

VFLYBACK = VSEC + (VIN - VOUT) x N

where: N is the transformer turns ratio SEC/PRI

VSEC is the maximum secondary DC output voltage

VOUT is the primary (main) output voltage

Subtract the main output voltage (VOUT) from VFLYBACK

in this equation if the secondary winding is returned to

VOUT and not to ground. The diode reverse breakdown

rating must also accommodate any ringing due to leak-

age inductance. D3’s current rating should be at least

twice the DC load current on the secondary output.

_____________Low-Voltage Operation

Low input voltages and low input-output differential volt-

ages each require some extra care in the design. Low

absolute input voltages can cause the VL linear regula-

tor to enter dropout, and eventually shut itself off. Low

input voltages relative to the output (low VIN-VOUT differ-

ential) can cause bad load regulation in multi-output fly-

back applications. See

Transformer Design section.

Finally, low VIN-VOUT differentials can also cause the

output voltage to sag when the load current changes

abruptly. The amplitude of the sag is a function of induc-

tor value and maximum duty factor (DMAX an Electrical

Characteristics parameter, 98% guaranteed over tem-

perature at f = 150kHz) as follows:

(ISTEP)2 x L

VSAG = ———————————————

2 x COUT x (VIN(MIN) x DMAX - VOUT)

The cure for low-voltage sag is to increase the value of

the output capacitor. For example, at VIN = 5.5V, VOUT

= 5V, L = 10µH, f = 150kHz, a total capacitance of

660µF will prevent excessive sag. Note that only the

capacitance requirement is increased and the ESR

requirements don’t change. Therefore, the added

capacitance can be supplied by a low-cost bulk

capacitor in parallel with the normal low-ESR capacitor.

Table 4 summarizes low-voltage operational issues.

Table 4. Low-Voltage Troubleshooting

Supply VL from an external source other

than VBATT, such as the system 5V supply.

VL output is so low that it hits the

VL UVLO threshold at 4.2V max.

Low input voltage, <4.5V

Won’t start under load or

quits before battery is

completely dead

Use a small 20mA Schottky diode for

boost diode D2. Supply VL from an

external source.

VL linear regulator is going into

dropout and isn’t providing

good gate-drive levels.

Low input voltage, <5V

High supply current,

poor efficiency

Reduce f to 150kHz. Reduce secondary

impedances—use Schottky if possible.

Stack secondary winding on main output.

Not enough duty cycle left to

initiate forward-mode operation.

Small AC current in primary can’t

store energy for flyback operation.

Low VIN-VOUT differential,

VIN < 1.3 x VOUT(main)

Secondary output won’t

support a load

Increase the minimum input voltage or

ignore.

Normal function of internal low-

dropout circuitry.

Low VIN-VOUT differential,

<0.5V

Unstable—jitters between

two distinct duty factors

Reduce f to 150kHz. Reduce MOSFET

on-resistance and coil DCR.

Maximum duty-cycle limits

exceeded.

Low VIN-VOUT differential,

<0.5V

Dropout voltage is too

high (VOUT follows VIN as

VIN decreases)

Increase bulk output capacitance per

formula above. Reduce inductor value.

Limited inductor-current slew

rate per cycle.

Low VIN-VOUT differential,

<1V

Sag or droop in VOUT

under step load change

SOLUTION

ROOT CAUSE

CONDITION

SYMPTOM

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