LTC3852EUDD#TRPBF Datasheet LTC3852IUDD#PBF, LTC3852EUDD#PBF, LTC3852EUDD#TRPBF | P13-P15

LTC3852

3852f

is recommended that there is always a load present during

the drop-out transition to ensure C

is recharged.

Shutdown and Start-Up (RUN, SHDN and TRACK/SS)

The switching regulator section of the LTC3852 can be

shut down using the RUN pin. Pulling this pin below 1.1V

disables the controller and most of the internal circuitry.

Releasing the RUN pin allows an internal 2µA current to

pull up the pin and enable the controller. Alternatively, the

RUN pin may be externally pulled up or driven directly by

logic. Be careful not to exceed the absolute maximum

rating of 6V on this pin.

The start-up of the controller’s output voltage, V

OUT

, is

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

voltage on the TRACK/SS pin is less than the 0.8V internal

reference, the LTC3852 regulates the V

voltage to the

TRACK/SS pin voltage instead of the 0.8V reference. This

allows the TRACK/SS pin to be used to program a soft-start

by connecting an external capacitor from the TRACK/SS

pin to GND. An internal 1µA pull-up current charges this

capacitor, creating a voltage ramp on the TRACK/SS pin.

As the TRACK/SS voltage rises linearly from 0V to 0.8V

(and beyond), the output voltage V

OUT

rises smoothly from

zero to its ﬁ nal value. Alternatively, the TRACK/SS pin can

be used to cause the start-up of V

OUT

to “track” another

supply. Typically, this requires connecting to the TRACK/SS

pin an external resistor divider from the other supply to

ground (see the Applica tions Information section). When

the RUN pin is pulled low to disable the controller, or when

INTV

drops below its undervoltage lockout threshold

of 3.2V, the TRACK/SS pin is pulled low by an internal

MOSFET. When in undervoltage lockout, the controller is

disabled and the external MOSFETs are held off.

The charge pump is separately controlled by SHDN. In

shutdown mode, all charge pump circuitry is turned off and it

draws only leakage current from the V

IN1

supply. Furthermore,

PUMP

is disconnected from V

IN1

. The SHDN pin is a CMOS

input with a threshold voltage of approximately 0.7V. The

charge pump is in shutdown when a logic low is applied to

the SHDN pin. Since the SHDN pin is a very high impedance

CMOS input, it should never be allowed to ﬂ oat. To ensure

that its state is deﬁ ned, it must always be driven with a valid

logic level not exceeding V

IN1

, even if it is tied to RUN.

OPERATION

Since the output voltage of the charge pump can go above

the input voltage, special circuitry is required to control the

internal logic. Detection logic will draw an input current

of 5µA when in shutdown. However, this current will

be eliminated if the output voltage (V

PUMP

) is less than

approximately 0.8V.

The charge pump has built-in soft-start circuitry to prevent

excessive current ﬂ ow during start-up. The soft-start is

achieved by charging an internal capacitor with a very weak

current source. The voltage on this capacitor, in turn, slowly

ramps the amount of current available to the output storage

capacitor from zero to a value of 50mA over a period of

approximately 125µs. The soft-start circuit is reset in the

event of a commanded shutdown or thermal shutdown.

Light Load Current Operation (Burst Mode Operation,

Pulse skipping or Continuous Conduction)

The LTC3852 can be enabled for high efﬁ ciency Burst

Mode operation, constant frequency pulse skipping mode

or forced continuous conduction mode. To select forced

continuous operation, tie the MODE/PLLIN pin to INTV

To select pulse skipping mode of operation, ﬂ oat the

MODE/PLLIN pin or tie it to GND2. To select Burst Mode

operation, tie MODE/PLLIN to INTV

through a resistor

no less than 50k, but no greater than 250k.

When the controller is enabled for Burst Mode operation,

the peak current in the inductor is set to approximately

one-fourth of the maximum sense voltage even though

the voltage on the I

pin indicates a lower value. If the

average inductor current is higher than the load current,

the error ampliﬁ er, EA, will decrease the voltage on the I

pin. When the I

voltage drops below 0.4V, the internal

sleep signal goes high (enabling sleep mode) and both

external MOSFETs are turned off.

In sleep mode, the load current is supplied by the output

capacitor. As the output voltage decreases, the EA’s output

begins to rise. When the output voltage drops enough, the

sleep signal goes low, and the controller resumes normal

operation by turning on the top external MOSFET on the

next cycle of the internal oscillator. When the controller is

enabled for Burst Mode operation, the inductor current is

not allowed to reverse. The reverse current comparator,

REV

, turns off the bottom external MOSFET just before the

LTC3852

3852f

inductor current reaches zero, preventing it from revers-

ing and going negative. Thus, the controller operates in

discontinuous operation. In forced continuous operation,

the inductor current is allowed to reverse at light loads

or under large transient conditions. The peak inductor

cur rent is determined by the voltage on the I

pin, just

as in normal operation. In this mode the efﬁ ciency at light

loads is lower than in Burst Mode operation. However,

continuous mode has the advantages of lower output

ripple and constant frequency operation.

When the MODE/PLLIN pin is connected to GND2, the

LTC3852 operates in PWM pulse skipping mode at light

loads. At very light loads the current comparator, I

CMP

, may

remain tripped for several cycles and force the external top

MOSFET to stay off for the same number of cycles (i.e.,

skipping pulses). The inductor current is not allowed to

reverse (discontinuous operation). This mode, like forced

continuous operation, exhibits low output ripple as well as

low audio noise and reduced RF interference as compared

to Burst Mode operation. It provides higher low current

efﬁ ciency than forced continuous mode, but not as high

as Burst Mode operation.

Frequency Selection and Phase-Locked Loop

(FREQ/PLLFLTR and MODE/PLLIN Pins)

The selection of a switching frequency is a trade-off between

efﬁ ciency and component size. Low frequency operation

increases efﬁ ciency by reducing MOSFET switching losses, but

requires larger inductance and/or capacitance to main tain low

output ripple voltage. The switching frequency of the LTC3852’s

controller can be selected using the FREQ/PLLFLTR pin. If

the MODE/PLLIN pin is not being driven by an external clock

source, the FREQ/PLLFLTR pin can be used to program the

controller’s operating frequency from 250kHz to 750kHz.

A phase-locked loop (PLL) is available on the LTC3852

to synchronize the internal oscillator to an external clock

source that is connected to the MODE/PLLIN pin. The

controller operates in forced continuous mode of operation

when it is synchronized. A series RC should be connected

between the FREQ/PLLFLTR pin and GND to serve as the

PLL’s loop ﬁ lter.

It is suggested that the external clock be applied before

enabling the controller unless a second resistor is

OPERATION

connected in parallel with the series RC loop ﬁ lter network.

The second resistor prevents low switching frequency

operation if the controller is enabled before the clock.

Output Overvoltage Protection

An overvoltage comparator, OV, guards against transient

overshoots (>10%) as well as other more serious con-

ditions that may overvoltage the output of the step-down

controller. In such cases, the top MOSFET is turned off

and the bottom MOSFET is turned on until the overvoltage

condition is cleared.

Power Good (PGOOD) Pin

The PGOOD pin is connected to an open drain of an internal

N-channel MOSFET. The MOSFET turns on and pulls the

PGOOD pin low when the V

pin voltage is not within

±10% of the 0.8V reference voltage. The PGOOD pin is

also pulled low when the RUN pin is low (shut down)

or when the LTC3852’s controller is in the soft-start or

tracking phase. When the V

pin voltage is within the

±10% requirement, the MOSFET is turned off and the

pin is allowed to be pulled up by an external resistor to

a source of up to 6V (abs max). The PGOOD pin will ﬂ ag

power good immediately when the V

pin is within the

±10% window. However, there is an internal 17µs power

bad mask when V

goes out of the ±10% window.

Short-Circuit/Thermal Protection

The charge pump has built-in short-circuit current limit as

well as over-temperature protection. During a short-circuit

condition, it will automatically limit V

PUMP

output current

to approximately 300mA. At higher temperatures, or if the

input voltage is high enough to cause excessive self-heating

of the part, the thermal shutdown circuitry will shut down

the charge pump once the junction temperature exceeds

approximately 160°C. It will enable the charge pump once its

junction temperature drops back to approximately 150°C.

The charge pump will cycle in and out of thermal shutdown

indeﬁ nitely until the short-circuit condition on V

PUMP

removed. The maximum rated junction temperature will

be exceeded when this thermal shutdown protection is

active. Continuous operation above the speciﬁ ed absolute

maximum operating junction temperature may impair

device reliability or permanently damage the device.

LTC3852

3852f

The Typical Application on the ﬁ rst page of this data sheet

is a basic LTC3852 application circuit. The LTC3852 can

be conﬁ gured to use either DCR (inductor resistance)

sensing or low value resistor sensing. The choice of the

two current sensing schemes is largely a design trade-off

between cost, power consumption and accuracy. DCR

sensing is popular because it saves expensive current

sensing resis tors and is more power efﬁ cient, especially

in high current applications. However, current sensing

resistors provide the most accurate current limits for the

controller. Other external component selection is driven

by the load require ment, and begins with the selection of

SENSE

(if R

SENSE

is used) and the inductor value. Next,

the power MOSFETs and Schottky diodes are selected.

Finally, input and output capacitors are selected.

SENSE

and SENSE

–

Pins

The SENSE

and SENSE

–

pins are the inputs to the current

comparators. The common mode input voltage range of

the current comparators is 0V to 5.5V. Both SENSE pins

are high impedance inputs with small base currents of

less than 1A. When the SENSE pins ramp up from 0V

to 1.4V, the small base currents ﬂ ow out of the SENSE

pins. When the SENSE pins ramp down from 5V to 1.1V,

the small base currents ﬂ ow into the SENSE pins. The

high impedance inputs to the current comparators allow

accurate DCR sensing. However, care must be taken not

to ﬂ oat these pins during normal operation.

Low Value Resistors Current Sensing

A typical sensing circuit using a discrete resistor is shown

in Figure 2. R

SENSE

is chosen based on the required

output current. For simplicity, the charge pump section

is omitted.

The current comparator has a maximum threshold,

MAX

= 50mV. The current comparator threshold sets the

maximum peak of the inductor current, yielding a maximum

average output current, I

MAX

, equal to the peak value less

APPLICATIONS INFORMATION

half the peak-to-peak ripple current, DI

. Allowing a margin

of 20% for variations in the IC and external component

values yields:

SENSE

= 0.8 •

MAX

+ΔI

IN2

INTV

BOOST

GND2

FILTER COMPONENTS

PLACED NEAR SENSE PINS

SENSE

–

LTC3852

OUT

SENSE

3852 F02

Inductor DCR Sensing

For applications requiring the highest possible efﬁ ciency,

the LTC3852 is capable of sensing the voltage drop across

the inductor DCR, as shown in Figure 3. The DCR of the

inductor represents the small amount of DC winding

resis tance of the copper, which can be less than 1mW for

today’s low value, high current inductors. If the external

R1||R2 • C1 time constant is chosen to be exactly equal

to the L/DCR time constant, the voltage drop across the

external capacitor is equal to the voltage drop across the

inductor DCR multiplied by R2/(R1 + R2). Therefore, R2

may be used to scale the voltage across the sense terminals

when the DCR is greater than the target sense resistance.

Check the manufacturer’s data sheet for speciﬁ cations

regarding the inductor DCR, in order to properly dimension

the external ﬁ lter components. The DCR of the inductor

can also be measured using a good RLC meter.

Figure 2. Using a Resistor to Sense Current with the LTC3852

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P27 P28-P30 P31-P32

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Switching Voltage Regulators Low Input Voltage Synchronous Step-Down Controller

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