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LTC1624

100

Ω resistor in series with the SENSE– pin. This offset

cancels the internal offset in current comparator I2 (refer

to Functional Diagram). This comparator in conjunction

with the voltage on the ITH/RUN pin determines when to

enter into Burst Mode operation (refer to Low Current

Operation in Operation section). With the additional exter-

nal offset present, the drive to the topside MOSFET is

always enabled every cycle and constant frequency opera-

tion occurs for IOUT > IOUT(MIN).

Step-Down Converter: Design Example

As a design example, assume VIN = 12V(nominal),

VIN = 22V(max), VOUT = 3.3V and IMAX = 2A. RSENSE can

immediately be calculated:

RSENSE = 100mV/2A = 0.05Ω

Assume a 10

µH inductor. To check the actual value of the

ripple current the following equation is used:

∆I

OUT

−

()( )









The highest value of the ripple current occurs at the

maximum input voltage:

∆I

kHz

L =

−

()









3 3

200

0 5

158

AP-P

The power dissipation on the topside MOSFET can be

easily estimated. Choosing a Siliconix Si4412DY results

in: RDS(ON) = 0.042Ω, CRSS = 100pF. At maximum input

voltage with T(estimated) = 50

°C:

kHz

MAIN =

() +() °− °

()

[]()

() ( )( )(

0 5

0 005 50

0 042

2 5 22

100

200

185

Ω

The most stringent requirement for the Schottky diode

occurs when VOUT= 0V (i.e. short circuit) at maximum VIN.

In this case the worst-case dissipation rises to:

SC AVG

()









()

APPLICATIONS INFORMATION

With the 0.05

Ω sense resistor ISC(AVG) = 2A will result,

increasing the 0.5V Schottky diode dissipation to 0.98W.

CIN is chosen for an RMS current rating of at least 1.0A at

temperature. COUT is chosen with an ESR of 0.03Ω for low

output ripple. The output ripple in continuous mode will be

highest at the maximum input voltage. The output voltage

ripple due to ESR is approximately:

VORIPPLE = RESR(∆IL) = 0.03Ω (1.58AP-P) = 47mVP-P

Step-Down Converter: Duty Cycle Limitations

At high input to output differential voltages the on-time

gets very small. Due to internal gate delays and response

times of the internal circuitry the minimum recommended

on-time is 450ns. Since the LTC1624’s frequency is inter-

nally set to 200kHz a potential duty cycle limitation exists.

When the duty cycle is less than 9%, cycle skipping may

occur which increases the inductor ripple current but does

not cause VOUT to lose regulation. Avoiding cycle skipping

imposes a limit on the input voltage for a given output

voltage only when VOUT < 2.2V using 30V MOSFETs.

(Remember not to exceed the absolute maximum voltage

of 36V.)

VIN(MAX) = 11.1VOUT + 5V

For DC > 9%

Boost Converter Applications

The LTC1624 is also well-suited to boost converter appli-

cations. A boost converter steps up the input voltage to a

higher voltage as shown in Figure 6.

Figure 6. Boost Converter

RSENSE

CIN

VIN

1624 F06

VIN

VFB

LTC1624

SENSE–

BOOST

GND

COUT

VOUT

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