LTC3602EFE#PBF Datasheet LTC3602IUF#PBF, LTC3602EUF#TRPBF, LTC3602EFE#TRPBF | P7-P9

LTC3602

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OPERATION

Main Control Loop

The LTC3602 is a monolithic, constant-frequency, current-

mode step-down DC/DC converter. During normal opera-

tion, the internal top power switch (N-channel MOSFET) is

turned on at the beginning of each clock cycle. Current in

the inductor increases until the current comparator trips

and turns off the top power MOSFET. The peak inductor

current at which the current comparator shuts off the top

power switch is controlled by the voltage on the I

pin.

The error ampliﬁ er adjusts the voltage on the I

pin by

comparing the feedback signal from a resistor divider on

the V

pin with an internal 0.6V reference. When the load

current increases, it causes a reduction in the feedback

voltage relative to the reference. The error ampliﬁ er raises

the I

voltage until the average inductor current matches

the new load current. When the top power MOSFET shuts

off, the synchronous power switch (N-channel MOSFET)

turns on until either the bottom current limit is reached or

the beginning of the next clock cycle. The bottom current

limit is set at –2.5A for forced continuous mode and 0A

for Burst Mode operation.

The operating frequency is externally set by an external

resistor connected between the R

pin and ground. The

practical switching frequency can range from 300kHz to

3MHz.

Overvoltage and undervoltage comparators will pull the

PGOOD output low if the output voltage comes out of

regulation by ±7.5%. In an overvoltage condition, the top

power MOSFET is turned off and the bottom power MOSFET

is switched on until either the overvoltage condition clears

or the bottom MOSFET’s current limit is reached.

Forced Continuous Mode

Connecting the SYNC/MODE pin to INTV

will disable Burst

Mode operation and force continuous current operation.

At light loads, forced continuous mode operation is less

efﬁ cient than Burst Mode operation, but may be desirable in

some applications where it is necessary to keep switching

harmonics out of a signal band. The output voltage ripple

is minimized in this mode.

Burst Mode Operation

Connecting the SYNC/MODE pin to a voltage in the range

of 0.42V to 1V enables Burst Mode operation. In Burst

Mode operation, the internal power MOSFETs operate

intermittently at light loads. This increases efﬁ ciency by

minimizing switching losses. During Burst Mode opera-

tion, the minimum peak inductor current is externally set

by the voltage on the SYNC/MODE pin and the voltage

on the I

pin is monitored by the burst comparator to

determine when sleep mode is enabled and disabled.

When the average inductor current is greater than the

load current, the voltage on the I

pin drops. As the I

voltage falls below 330mV, the burst comparator trips and

enables sleep mode. During sleep mode, the top power

MOSFET is held off and the I

pin is disconnected from

the output of the error ampliﬁ er. The majority of the internal

circuitry is also turned off to reduce the quiescent current

to 75μA while the load current is solely supplied by the

output capacitor. When the output voltage drops, the I

pin is reconnected to the output of the error ampliﬁ er and

the top power MOSFET along with all the internal circuitry

is switched back on. This process repeats at a rate that

is dependent on the load demand. Pulse-skipping opera-

tion is implemented by connecting the SYNC/MODE pin

to ground. This forces the burst clamp level to be at 0V.

As the load current decreases, the peak inductor current

will be determined by the voltage on the I

pin until the

voltage drops below 330mV. At this point, the peak

inductor current is determined by the minimum on-time

of the current comparator. If the load demand is less than

the average of the minimum on-time inductor current,

switching cycles will be skipped to keep the output volt-

age in regulation.

Frequency Synchronization

The internal oscillator of the LTC3602 can be synchronized

to an external clock connected to the SYNC/MODE pin.

The frequency of the external clock can be in the range of

300kHz to 3MHz. For this application, the oscillator timing

resistor should be chosen to correspond to a frequency

that is 25% lower than the synchronization frequency.

When synchronized, the LTC3602 will operate in pulse-

skipping mode.

LTC3602

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OPERATION

Dropout Operation

When the input supply voltage decreases toward the output

voltage, the duty cycle increases toward the maximum

on-time. Further reduction of the supply voltage forces the

top switch to remain on for more than one cycle until it

attempts to stay on continuously. In order to replenish the

voltage on the ﬂ oating BOOST supply capacitor, however,

the top switch is forced off and the bottom switch is forced

on for approximately 85ns every sixteen clock cycles. This

achieves an effective duty cycle that can exceed 99%. The

output voltage will then be primarily determined by the

input voltage minus the voltage drop across the upper

internal N-channel MOSFET and the inductor.

Slope Compensation and Inductor Peak Current

Slope compensation provides stability in constant-fre-

quency architectures by preventing subharmonic oscilla-

tions at duty cycles greater than 50%. It is accomplished

internally by adding a compensating ramp to the inductor

current signal at duty cycles in excess of 30%. Normally,

the maximum inductor peak current is reduced when

slope compensation is added. In the LTC3602, however,

slope compensation recovery is implemented to reduce

the variation of the maximum inductor peak current (and

therefore the maximum available output current) over the

range of duty cycles.

Short-Circuit Protection

When the output is shorted to ground, the inductor cur-

rent decays very slowly during a single switching cycle.

To prevent current runaway from occurring, a secondary

current limit is imposed on the inductor current. If the

inductor valley current increases to more than 4.5A, the

top power MOSFET will be held off and switching cycles

will be skipped until the inductor current is reduced.

Overtemperature Protection

When using the LTC3602 in an application circuit, care

must be taken not to exceed any of the ratings speci-

ﬁ ed in the Absolute Maximum Ratings section. As an

added safeguard, however, the LTC3602 does incorporate

an overtemperature shutdown feature. If the junction

temperature reaches approximately 150°C, both power

switches will be turned off and the SW node will become

high impedance. After the part has cooled to below 115°C,

it will restart.

Voltage Tracking and Soft-Start

Some microprocessors and DSP chips need two power

supplies with different voltage levels. These systems often

require voltage sequencing between the core power supply

and the I/O power supply. Without proper sequencing,

latch-up failure or excessive current draw may occur that

could result in damage to the processor’s I/O ports or the

I/O ports of a supporting system device such as memory,

an FPGA or a data converter. To ensure that the I/O loads

are not driven until the core voltage is properly biased,

tracking of the core supply and the I/O supply voltage is

necessary.

Voltage tracking is enabled by applying a ramp voltage to

the TRACK/SS pin. When the voltage on the TRACK pin

is below 0.6V, the feedback voltage will regulate to this

tracking voltage. When the tracking voltage exceeds 0.6V,

tracking is disabled and the feedback voltage will regulate

to the internal reference voltage.

The TRACK/SS pin is also used to implement an external

soft-start function. A 1.2μA current is sourced from this

pin so that an external capacitor may be added to create

a smooth ramp. If this ramp is slower than the internal

1ms soft-start, then the output voltage will track this ramp

during start up instead. Leave this pin ﬂ oating to use the

internal 1ms soft-start ramp. Do not tie the TRACK/SS

pin to INTV

or to PV

LTC3602

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APPLICATIONS INFORMATION

The basic LTC3602 application circuit is shown on the front

page of this data sheet. External component selection is

determined by the maximum load current and begins with

the selection of the inductor value and operating frequency

followed by C

and C

OUT

Operating Frequency

Selection of the operating frequency is a tradeoff between

efﬁ ciency and component size. High frequency operation

allows the use of smaller inductor and capacitor values.

Operation at lower frequencies improves efﬁ ciency by

reducing internal gate charge and switching losses but

requires larger inductance values and/or capacitance to

maintain low output ripple voltage. The operating frequency

of the LTC3602 is determined by an external resistor that is

connected between the R

pin and ground. The value of the

resistor sets the ramp current that is used to charge and

discharge an internal timing capacitor within the oscillator

and can be calculated by using the following equation:

fHz

OSC

115 10

.•

()

–

Although frequencies as high as 3MHz are possible, the

minimum on-time of the LTC3602 imposes a minimum

limit on the operating duty cycle. The minimum on-time

is typically 90ns. Therefore, the minimum duty cycle is

equal to 100 • 90ns • f(Hz).

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current ΔI

increases with higher V

and decreases

with higher inductance.

ΔI

OUT

⎛

⎝

⎜

⎞

⎠

⎟

•1–

OUT

⎛

⎝

⎜

⎞

⎠

⎟

Having a lower ripple current reduces the ESR losses

in the output capacitors and the output voltage ripple.

Highest efﬁ ciency operation is achieved at low frequency

with small ripple current. This, however, requires a large

inductor.

A reasonable starting point for selecting the ripple current

is ΔI

= 0.4(I

MAX

), where I

MAX

is the maximum output

current. The largest ripple current occurs at the highest

. To guarantee that the ripple current stays below a

speciﬁ ed maximum, the inductor value should be chosen

according to the following equation:

L =

OUT

fΔI

L(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

•1–

OUT

IN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

The inductor value will also have an effect on Burst Mode

operation. The transition from low current operation

begins when the peak inductor current falls below a level

set by the burst clamp. Lower inductor values result in

higher ripple current which causes this to occur at lower

load currents. This causes a dip in efﬁ ciency in the upper

range of low current operation. In Burst Mode operation,

lower inductance values will cause the burst frequency

to increase.

Inductor Core Selection

Once the value for L is known, the type of inductor must

be selected. High efﬁ ciency converters generally cannot

afford the core loss found in low cost powdered iron cores,

forcing the use of the more expensive ferrite cores. Actual

core loss is independent of core size for a ﬁ xed inductor

value but it is very dependent on the inductance selected.

As the inductance increases, core losses decrease. Un-

fortunately, increased inductance requires more turns of

wire and therefore copper losses will increase.

Ferrite designs have very low core losses and are pre-

ferred at high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation.

Ferrite core material saturates “hard,” which means that

inductance collapses abruptly when the peak design current

is exceeded. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple. Do

not allow the core to saturate!

Different core materials and shapes will change the

size/current and price/current relationship of an inductor.

Toroid or shielded pot cores in ferrite or permalloy ma-

terials are small and do not radiate energy but generally

cost more than powdered iron core inductors with similar

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LTC3602EFE#PBF

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Description:

Switching Voltage Regulators 2.5A, 10V, Mono Sync Buck Reg

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