LTC3809EDD-1#PBF Datasheet LTC3809EDD-1#PBF, LTC3809IMSE-1#PBF, LTC3809IDD-1#PBF | P10-P12

LTC3809-1

38091fc

In sleep mode, much of the internal circuitry is turned off,

reducing the quiescent current that the LTC3809-1 draws.

The load current is supplied by the output capacitor. As

the output voltage decreases, the EAMP increases the

voltage. When the I

voltage reaches 0.925V, the

SLEEP signal goes low and the controller resumes normal

operation by turning on the external P-channel MOSFET

on the next cycle of the internal oscillator.

When the controller is enabled for Burst Mode or pulse-

skipping operation, the inductor current is not allowed to

reverse. Hence, the controller operates discontinuously.

The reverse current comparator RICMP senses the

drain-to-source voltage of the bottom external N-channel

MOSFET. This MOSFET is turned off just before the inductor

current reaches zero, preventing it from going negative.

In forced continuous operation, the inductor current is

allowed to reverse at light loads or under large transient

conditions. The peak inductor current is determined by the

voltage on the I

pin. The P-channel MOSFET is turned

on every cycle (constant frequency) regardless of the I

pin voltage. 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 no

noise at audio frequencies.

When the MODE pin is set to the V

Pin, the LTC3809-1

operates in PWM pulse-skipping mode at light loads. In

this mode, the current comparator ICMP may remain

tripped for several cycles and force the external P-channel

MOSFET to stay off for the same number of cycles. The

inductor current is not allowed to reverse (discontinuous

operation). This mode, like forced continuous operation,

exhibits low output ripple as well as low audible noise

and reduced RF interference as compared to Burst Mode

operation. However, it provides low current efﬁ ciency

higher than forced continuous mode, but not nearly as

high as Burst Mode operation. During start-up or an

undervoltage condition (V

≤ 0.54V), the LTC3809-1

operates in pulse-skipping mode (no current reversal

allowed), regardless of the state of the MODE pin.

Short-Circuit and Current Limit Protection

The LTC3809-1 monitors the voltage drop ΔV

(between

the GND and SW pins) across the external N-channel

MOSFET with the short-circuit current limit comparator.

The allowed voltage is determined by:

ΔV

SC(MAX)

= A • 90mV

where A is a constant determined by the state of the IPRG

pin. Floating the IPRG pin selects A = 1; tying IPRG to V

selects A = 5/3; tying IPRG to GND selects A = 2/3.

The inductor current limit for short-circuit protection is

determined by ΔV

SC(MAX)

and the on-resistance of the

external N-channel MOSFET:

SC MAX

DS ON

()

Once the inductor current exceeds I

, the short current

comparator will shut off the external P-channel MOSFET

until the inductor current drops below I

Output Overvoltage Protection

As further protection, the overvoltage comparator (OVP)

guards against transient overshoots, as well as other more

serious conditions that may overvoltage the output. When

the feedback voltage on the V

pin has risen 13.33%

above the reference voltage of 0.6V, the external P-channel

MOSFET is turned off and the N-channel MOSFET is turned

on until the overvoltage is cleared.

OPERATION

(Refer to Functional Diagram)

LTC3809-1

38091fc

Dropout Operation

When the input supply voltage (V

) approaches the output

voltage, the rate of change of the inductor current while the

external P-channel MOSFET is on (ON cycle) decreases.

This reduction means that the P-channel MOSFET will

remain on for more than one oscillator cycle if the inductor

current has not ramped up to the threshold set by the

EAMP on the I

pin. Further reduction in the input supply

voltage will eventually cause the P-channel MOSFET to be

turned on 100%; i.e., DC. The output voltage will then be

determined by the input voltage minus the voltage drop

across the P-channel MOSFET and the inductor.

Undervoltage Lockout

To prevent operation of the P-channel MOSFET below

safe input voltage levels, an undervoltage lockout is

incorporated in the LTC3809-1. When the input supply

voltage (V

) drops below 2.25V, the external P- and

N-channel MOSFETs and all internal circuits are turned

off except for the undervoltage block, which draws only

a few microamperes.

Peak Current Sense Voltage Selection

and Slope Compensation (IPRG Pin)

When the LTC3809-1 controller is operating below 20%

duty cycle, the peak current sense voltage (between the

and SW pins) allowed across the external P-channel

MOSFET is determined by:

Δ=VA

SENSE MAX

ITH

()

•

–.07

where A is a constant determined by the state of the IPRG

pin. Floating the IPRG pin selects A = 1; tying IPRG to

selects A = 5/3; tying IPRG to GND selects A = 2/3.

The maximum value of V

ITH

is typically about 1.98V, so

the maximum sense voltage allowed across the external

P-channel MOSFET is 125mV, 85mV or 204mV for the

three respective states of the IPRG pin.

However, once the controller’s duty cycle exceeds 20%,

slope compensation begins and effectively reduces the

peak sense voltage by a scale factor (SF) given by the

curve in Figure 1.

The peak inductor current is determined by the peak sense

voltage and the on-resistance of the external P-channel

MOSFET:

SENSE MAX

DS ON

()

OPERATION

(Refer to Functional Diagram)

Figure 1. Maximum Peak Current vs Duty Cycle

DUTY CYCLE (%)

SF = I/I

MAX

(%)

110

100

38091 F01

200

100

LTC3809-1

38091fc

The typical LTC3809-1 application circuit is shown in Figure

8. External component selection for the controller is driven

by the load requirement and begins with the selection of

the inductor and the power MOSFETs.

Power MOSFET Selection

The LTC3809-1’s controller requires two external power

MOSFETs: a P-channel MOSFET for the topside (main)

switch and a N-channel MOSFET for the bottom (synchro-

nous) switch. The main selection criteria for the power

MOSFETs are the breakdown voltage V

BR(DSS)

, threshold

voltage V

GS(TH)

, on-resistance R

DS(ON)

, reverse transfer

capacitance C

RSS

, turn-off delay t

D(OFF)

and the total gate

charge Q

The gate drive voltage is the input supply voltage. Since

the LTC3809-1 is designed for operation down to low input

voltages, a sublogic level MOSFET (R

DS(ON)

guaranteed at

= 2.5V) is required for applications that work close to

this voltage. When these MOSFETs are used, make sure that

the input supply to the LTC3809-1 is less than the absolute

maximum MOSFET V

rating, which is typically 8V.

The P-channel MOSFET’s on-resistance is chosen based

on the required load current. The maximum average load

current I

OUT(MAX)

is equal to the peak inductor current

minus half the peak-to-peak ripple current I

RIPPLE

. The

LTC3809-1’s current comparator monitors the drain-to-

source voltage V

of the top P-channel MOSFET, which

is sensed between the V

and SW pins. The peak inductor

current is limited by the current threshold, set by the voltage

on the I

pin, of the current comparator. The voltage on

the I

pin is internally clamped, which limits the maximum

current sense threshold ΔV

SENSE(MAX)

to approximately

125mV when IPRG is ﬂ oating (85mV when IPRG is tied

low; 204mV when IPRG is tied high).

The output current that the LTC3809-1 can provide is

given by:

OUT MAX

SENSE MAX

DS ON

RIPPLE

()

–=

APPLICATIONS INFORMATION

where I

RIPPLE

is the inductor peak-to-peak ripple current

(see Inductor Value Calculation).

A reasonable starting point is setting ripple current I

RIPPLE

to be 40% of I

OUT(MAX)

. Rearranging the above equation

yields:

for Duty C

DS ON MAX

SENSE MAX

OUT MAX

()

•=

yycle <20%

However, for operation above 20% duty cycle, slope

compensation has to be taken into consideration to select

the appropriate value of R

DS(ON)

to provide the required

amount of load current:

RSF

DS ON MAX

SENSE MAX

OUT MAX

()

••=

where SF is a scale factor whose value is obtained from

the curve in Figure 1.

These must be further derated to take into account the

signiﬁ cant variation in on-resistance with temperature. The

following equation is a good guide for determining the re-

quired R

DS(ON)MAX

at 25°C (manufacturer’s speciﬁ cation),

allowing some margin for variations in the LTC3809-1 and

external component values:

RSF

DS ON MAX

SENSE MAX

OUT MAX

()

•.• •

•

ρρ

The ρ

is a normalizing term accounting for the temperature

variation in on-resistance, which is typically about 0.4%/°C,

as shown in Figure 2. Junction-to-case temperature T

about 10°C in most applications. For a maximum ambi-

ent temperature of 70°C, using ρ

80°C

~ 1.3 in the above

equation is a reasonable choice.

The N-channel MOSFET’s on resistance is chosen based

on the short-circuit current limit (I

). The LTC3809-

1’s short-circuit current limit comparator monitors the

drain-to-source voltage V

of the bottom N-channel

MOSFET, which is sensed between the GND and SW pins.

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Manufacturer:

Analog Devices / Linear Technology

Description:

Switching Voltage Regulators No Rsense, Low EMI DC/DC Controller in DFN

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