MAX1540A Datasheet, PDF(40/49 Page) Maxim Integrated Products – Dual Step-Down Controllers with Saturation Protection, Dynamic Output, and Linear Regulator

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MAX1540A Datasheet, PDF (40/49 Pages) Maxim Integrated Products – Dual Step-Down Controllers with Saturation Protection, Dynamic Output, and Linear Regulator

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Dual Step-Down Controllers with Saturation

Protection, Dynamic Output, and Linear Regulator

Power-MOSFET Dissipation

Worst-case conduction losses occur at the duty factor

extremes. For the high-side MOSFET (NH), the worst-

case power dissipation due to resistance occurs at

minimum input voltage:

(NH

Resistance)

ââ

VOUT

VIN

â â

(ILOAD )2

RDS(ON)

Generally, use a small high-side MOSFET to reduce

switching losses at high input voltages. However, the

RDS(ON) required to stay within package power-dissi-

pation limits often restricts how small the MOSFET can

be. The optimum occurs when the switching losses

equal the conduction (RDS(ON)) losses. High-side

switching losses do not become an issue until the input

is greater than approximately 15V.

Calculating the power dissipation in high-side

MOSFETs (NH) due to switching losses is difficult, since

it must allow for difficult-to-quantify factors that influ-

ence the turn-on and turn-off times. These factors

include the internal gate resistance, gate charge,

threshold voltage, source inductance, and PC board

layout characteristics. The following switching loss cal-

culation provides only a very rough estimate and is no

substitute for breadboard evaluation, preferably includ-

ing verification using a thermocouple mounted on NH:

( )2

PD (NH Switching) = VIN(MAX)

CRSS Ã fSW Ã ILOAD

IGATE

where CRSS is the reverse transfer capacitance of NH,

and IGATE is the peak gate-drive source/sink current

(1A typ).

Switching losses in the high-side MOSFET can become

a heat problem when maximum AC adapter voltages

are applied due to the squared term in the switching-

loss equation (C â VIN2 â fSW). If the high-side MOS-

FET chosen for adequate RDS(ON) at low-battery

voltages becomes extraordinarily hot when subjected

to VIN(MAX), consider choosing another MOSFET with

lower parasitic capacitance.

For the low-side MOSFET (NL), the worst-case power

dissipation always occurs at maximum battery voltage:

(NL

Resistance)

â¡

â¢1-

â£â¢

VOUT

VIN(MAX)

â¤

â¥

â¦â¥

(ILOAD

Ã RDS(ON)

The absolute worst case for MOSFET power dissipation

occurs under heavy overload conditions that are

greater than ILOAD(MAX) but are not high enough to

exceed the current limit and cause the fault latch to trip.

To protect against this possibility, âoverdesignâ the cir-

cuit to tolerate:

ILOAD

IVALLEY(MAX)

ââ

VOUT (VIN

2VIN

â VOUT

fSW L

)

â â

where IVALLEY(MAX) is the maximum valley current

allowed by the current-limit circuit, including threshold

tolerance and sense-resistance variation. The

MOSFETs must have a relatively large heatsink to han-

dle the overload power dissipation.

Choose a Schottky diode (DL) with a forward-voltage

drop low enough to prevent the low-side MOSFETâs

body diode from turning on during the dead time. As a

general rule, select a diode with a DC current rating

equal to 1/3 the load current. This diode is optional and

can be removed if efficiency is not critical.

Applications Information

Step-Down Converter Dropout

Performance

The output-voltage adjustable range for continuous-

conduction operation is restricted by the nonadjustable

minimum off-time one-shot. For best dropout perfor-

mance, use the slower (200kHz) on-time setting. When

working with low input voltages, the duty-factor limit

must be calculated using worst-case values for on- and

off-times. Manufacturing tolerances and internal propa-

gation delays introduce an error to the TON K-factor.

This error is greater at higher frequencies (Table 3).

Also, keep in mind that transient-response performance

of buck regulators operated too close to dropout is

poor, and bulk output capacitance must often be

added (see the VSAG equation in the Design Procedure

section).

The absolute point of dropout is when the inductor cur-

rent ramps down during the minimum off-time (ÎIDOWN)

as much as it ramps up during the on-time (ÎIUP). The

ratio h = ÎIUP/ÎIDOWN indicates the controllerâs ability

to slew the inductor current higher in response to

increased load, and must always be greater than 1. As

h approaches 1, the absolute minimum dropout point,

the inductor current cannot increase as much during

each switching cycle, and VSAG greatly increases

unless additional output capacitance is used.

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