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LT1619

Low Voltage Current Mode PWM Controller

Linear Technology 

LT1619

APPLICATIO S I FOR ATIO

Power MOSFET

MOSFET power dissipation can be separated into fre-

quency independent and frequency dependent compo-

nents. The RDS(ON) loss in the switch is the product of the

mean square switch current and switch RDS(ON) and it

does not vary with the operating frequency.

The frequency-dependent switching losses consist of 1)

switch transition loss due to finite rise and fall times of the

drain source voltage and the drain current 2) gate switch-

ing loss, i.e., a packet of charge Qg (the total gate charge)

which is moved from the gate drive power supply to

ground in every switch cycle, and 3) the drain switching

loss, charge stored on the parasitic drain capacitance,

COSS is dumped to ground as the switch is turned on. The

transistor loss can be expressed as:

PLOSS = IDRMS2 RDS(ON) + transition loss + QgVGfS

+ 1/2COSSVDS(OFF)2fS

where the transition loss can be estimated with:

2

Transition

Loss

=

ID

CRSSVDS(OFF)

IG(AVG)

fS

Qg = The total gate charge

VG = Gate drive voltage ≈ VDRV

IG(AVG) = The average MOSFET buffer output current

fS = Operating frequency

CRSS = The average CGD between VDS = 0V

and VDS = VDS(OFF)

At low VDS(OFF) (≤12V) and operating frequencies below

500kHz, the ohmic losses often dominate. For high voltage

converters, the transition loss and COSS charge dumping

loss can dramatically impact the converter efficiency.

MOSFETs with lower parasitic capacitances but higher

RDS(ON) may actually provide better efficiency in these

situations.

Capacitors

In a switch mode DC/DC converter, output ripple voltage

is the product of the equivalent series resistance (ESR) of

the output capacitor and the peak-to-peak capacitor

current. Depending on topology, current feeding the out-

put capacitor can be continuous or discontinuous. The input

current can also be continuous or discontinuous even if the

inductor current itself is continuous. In boost topology, the

inductor is in series with the input source so the input

current is continuous and the output current is discontinu-

ous. In buck-boost or flyback converters, the inductor is

not in series with the input source nor the output, so nei-

ther the input current nor output current is continuous.

Whenever a terminal current is discontinuous, the capaci-

tor at that terminal should be chosen to handle the ripple

current. Capacitor reliability will be adversely affected if

the ripple current exceeds the maximum allowable rat-

ings. This maximum rating is specified as the RMS ripple

current. Several capacitors may be mounted in parallel to

meet the size and ripple current requirements.

Besides the ripple voltage requirements, the output ca-

pacitor also needs to be sized for acceptable output

voltage variation under load transients.

Current Sensing Resistor RSENSE

The LT1619 drives a low side N-channel MOSFET switch.

The switch current is sensed with an external resistor

RSENSE connected between the source of the MOSFET and

ground. The internal blanking circuit blocks the voltage

spike developed across RSENSE for 280ns at switch turn-

on. The switch is turned off when the instantaneous

voltage across RSENSE exceeds the current limit threshold,

VSENSE. Allowing variations in VSENSE yields:

RSENSE

=

VSENSE(MIN)

IL(MAX)

The current limit threshold is constant and does not vary

with duty ratio.

Due to low signal level of the sense voltage, low inductance

sense resistors are required to reduce switching noise.

Low TC resistors maintain constant current limit over

temperature. Dale WSL and IRC series sense resistors

meet these criteria.

8

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