MAX8597 Datasheet, PDF(18/24 Page) Maxim Integrated Products – Low-Dropout, Wide-Input-Voltage, Step-Down Controllers

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

MAX8597 Datasheet, PDF (18/24 Pages) Maxim Integrated Products – Low-Dropout, Wide-Input-Voltage, Step-Down Controllers

◁

Low-Dropout, Wide-Input-Voltage,

Step-Down Controllers

where RDH is the high-side MOSFET driverâs average

on-resistance (1.25â¦ typ) and RGATE is the internal

gate resistance of the MOSFET (typically 0.5â¦ to 2â¦):

PHSDR = Qgs x VGS x fS x RGATE / (RGATE + RDH)

where VGS ~ VVL = 5V.

In addition to the losses above, add approximately

20% more for additional losses due to MOSFET output

capacitances and low-side MOSFET body-diode

reverse-recovery charge dissipated in the high-side

MOSFET that exists, but is not well defined in the

MOSFET data sheet. Refer to the MOSFET data sheet

for thermal-resistance specification to calculate the

PC board area needed to maintain the desired maxi-

mum operating junction temperature with the above-

calculated power dissipation. To reduce EMI caused

by switching noise, add a 0.1ÂµF or larger ceramic

capacitor from the high-side switch drain to the low-

side switch source or add resistors in series with DH

and DL to slow down the switching transitions.

However, adding a series resistor increases the power

dissipation of the MOSFETs, so be sure this does not

overheat the MOSFETs. The minimum load current must

exceed the high-side MOSFETâs maximum leakage plus

the maximum LX bias current over temperature.

Setting the Current-Limit

The MAX8597/MAX8598/MAX8599 controllers sense

the peak inductor current to provide constant-current

and hiccup current limit. The peak current-limit thresh-

old is set by an external resistor (R2 in Figure 1) togeth-

er with the internal current sink of 200ÂµA. The voltage

drop across the resistor R2 due to the 200ÂµA current

sets the maximum peak inductor current that can flow

through the high-side MOSFET or the optional current-

sense resistor (between the high-side MOSFET source

and LX) by the equations below:

IPEAK(MAX) = 200ÂµA x R2 / RDSON(HSFET)

IPEAK(MAX) = 200ÂµA x R2 / RSENSE

The actual corresponding maximum load current is

lower than the IPEAK(MAX) by half of the inductor ripple

current. If the RDS(ON) of the high-side MOSFET is used

for current sensing, use the maximum RDS(ON) at the

highest operating junction temperature to avoid false

tripping of the current limit at elevated temperature.

Consideration should also be given to the tolerance of

the 200ÂµA current sink. When the RDS(ON) of the high-

side MOSFET is used for current sensing, ringing on

the LX voltage waveform can interfere with the current

limit. Below is the procedure for selecting the value of the

series RC snubber circuit (R14 and C14 in Figure 1):

1) Connect a scope probe to measure VLX to GND,

and observe the ringing frequency, fR.

2) Find the capacitor value (connected from LX to

GND) that reduces the ringing frequency by half.

The circuit parasitic capacitance (CPAR) at LX is

then equal to 1/3 the value of the added capaci-

tance above. The circuit parasitic inductance (LPAR)

is calculated by:

( ) LPAR =

2Ï Ã fR 2 Ã CPAR

The resistor for critical dampening (R14) is equal to 2Ï x

fR x LPAR. Adjust the resistor value up or down to tailor

the desired damping and the peak voltage excursion.

The capacitor (C14) should be at least 2 to 4 times the

value of the CPAR in order to be effective. The power

loss of the snubber circuit is dissipated in the resistor

(R14) and is calculated as:

PR14 = C14 x (VIN)2 x fS

where VIN is the input voltage and fS is the switching

frequency. Choose an R14 power rating that meets the

specific applicationâs derating rule for the power dissi-

pation calculated.

Additionally, there is parasitic inductance of the cur-

rent-sensing element, whether the high-side MOSFET

(LSENSE_FET) or the optional current-sense resistor

(LRSENSE) are used, which is in series with the output

filter inductor. This parasitic inductance, together with

the output inductor, forms an inductive divider and

causes error in the current-sensing voltage. To com-

pensate for this error, a series RC circuit can be added

in parallel with the sensing element (see Figure 5). The

RC time constant should equal LRSENSE / RSENSE, or

LSENSE_FET / RDS(ON). First, set the value of R equal to or

less than R2 / 100. Then, the value of C is calculated as:

C = LRSENSE / (RSENSE x R) or

C = LSENSE_FET / (RDS(ON) x R)

Any PC board trace inductance in series with the sens-

ing element and output inductor should be added to

the specified FET or resistor inductance per the

respective manufacturerâs data sheet. For the case of

18 ______________________________________________________________________________________

▷