MAX1652EEE datasheet(22/28 Pages) MAXIM

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High-Efficiency, PWM, Step-Down

DC-DC Controllers in 16-Pin QSOP

______________________________________________________________________________________

Power from the main and secondary outputs is lumped

together to obtain an equivalent current referred to the

main output voltage (see

Inductor Value section for def-

initions of parameters). Set the value of the current-

sense resistor at 80mV / ITOTAL.

PTOTAL = the sum of the output power from

all outputs

ITOTAL = PTOTAL / VOUT = the equivalent output

current referred to VOUT

VOUT (VIN(MAX) - VOUT)

L(primary) = —————————————

VIN(MAX) x f x ITOTAL x LIR

VSEC + VFWD

Turns Ratio N = ——————————————

VOUT(MIN) + VRECT + VSENSE

where: VSEC is the minimum required rectified

secondary-output voltage

VFWD is the forward drop across the

secondary rectifier

VOUT(MIN) is the minimum value of the main

output voltage (from the

Electrical

Characteristics)

VRECT is the on-state voltage drop across the

synchronous-rectifier MOSFET

VSENSE is the voltage drop across the sense

resistor

In positive-output (MAX1652) applications, the trans-

former secondary return is often referred to the main

output voltage rather than to ground in order to reduce

the needed turns ratio. In this case, the main output

voltage must first be subtracted from the secondary

voltage to obtain VSEC.

______Selecting Other Components

MOSFET Switches

The two high-current N-channel MOSFETs must be

logic-level types with guaranteed on-resistance specifi-

cations at VGS = 4.5V. Lower gate threshold specs are

better (i.e., 2V max rather than 3V max). Drain-source

breakdown voltage ratings must at least equal the max-

imum input voltage, preferably with a 20% derating

factor. The best MOSFETs will have the lowest on-resis-

tance per nanocoulomb of gate charge. Multiplying

RDS(ON) x QG provides a meaningful figure by which to

compare various MOSFETs. Newer MOSFET process

technologies with dense cell structures generally give

the best performance. The internal gate drivers can tol-

erate more than 100nC total gate charge, but 70nC is a

more practical upper limit to maintain best switching

times.

In high-current applications, MOSFET package power

dissipation often becomes a dominant design factor.

I2R losses are distributed between Q1 and Q2 accord-

ing to duty factor (see the equations below). Switching

losses affect the upper MOSFET only, since the

Schottky rectifier clamps the switching node before the

synchronous rectifier turns on. Gate-charge losses are

dissipated by the driver and don’t heat the MOSFET.

Ensure that both MOSFETs are within their maximum

junction temperature at high ambient temperature by

calculating the temperature rise according to package

thermal-resistance specifications. The worst-case dissi-

pation for the high-side MOSFET occurs at the minimum

battery voltage, and the worst-case for the low-side

MOSFET occurs at the maximum battery voltage.

PD (upper FET) = ILOAD2 x RDS(ON) x DUTY

VIN x CRSS

+ VIN x ILOAD x f x

(––––––––––– +20ns)

IGATE

PD (lower FET) = ILOAD2 x RDS(ON) x (1 - DUTY)

DUTY = (VOUT + VQ2) / (VIN - VQ1 + VQ2)

where the on-state voltage drop VQ_ = ILOAD x RDS(ON)

CRSS = MOSFET reverse transfer capacitance

IGATE = DH driver peak output current capability

(1A typically)

20ns = DH driver inherent rise/fall time

Under output short circuit, the synchronous-rectifier

MOSFET suffers extra stress and may need to be over-

sized if a continuous DC short circuit must be tolerated.

During short circuit, Q2’s duty factor can increase to

greater than 0.9 according to:

Q2 DUTY (short circuit) = 1 - [VQ2 / (VIN(MAX) - VQ1 + VQ2)]

where the on-state voltage drop VQ = (120mV / RSENSE)

x RDS(ON).

Rectifier Diode D1

Rectifier D1 is a clamp that catches the negative induc-

tor swing during the 60ns dead time between turning

off the high-side MOSFET and turning on the low-side.

D1 must be a Schottky type in order to prevent the

lossy parasitic MOSFET body diode from conducting. It

is acceptable to omit D1 and let the body diode clamp

the negative inductor swing, but efficiency will drop one

or two percent as a result. Use an MBR0530 (500mA

rated) type for loads up to 1.5A, a 1N5819 type for

loads up to 3A, or a 1N5822 type for loads up to 10A.

D1’s rated reverse breakdown voltage must be at least

equal to the maximum input voltage, preferably with a

20% derating factor.

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