LTC3736EGN#PBF Datasheet LTC3736EGN#TRPBF, LTC3736EUF#TRPBF, LTC3736EUF#PBF | P10-P12

LTC3736

3736fa

Main Control Loop

The LTC3736 uses a constant frequency, current mode

architecture with the two controllers operating 180 de-

grees out of phase. During normal operation, the top

external P-channel power MOSFET is turned on when the

clock for that channel sets the RS latch, and turned off

when the current comparator (I

CMP

) resets the latch. The

peak inductor current at which I

CMP

resets the RS latch is

determined by the voltage on the I

pin, which is driven

by the output of the error amplifier (EAMP). The V

receives the output voltage feedback signal from an exter-

nal resistor divider. This feedback signal is compared to

the internal 0.6V reference voltage by the EAMP. When the

load current increases, it causes a slight decrease in V

relative to the 0.6V reference, which in turn causes the I

voltage to increase until the average inductor current

matches the new load current. While the top P-channel

MOSFET is off, the bottom N-channel MOSFET is turned

on until either the inductor current starts to reverse, as

indicated by the current reversal comparator, I

RCMP

, or the

beginning of the next cycle.

Shutdown, Soft-Start and Tracking Start-Up

(RUN/SS and TRACK Pins)

The LTC3736 is shut down by pulling the RUN/SS pin low.

In shutdown, all controller functions are disabled and the

chip draws only 9µA. The TG outputs are held high (off)

and the BG outputs low (off) in shutdown. Releasing

RUN/SS allows an internal 0.7µA current source to charge

up the RUN/SS pin. When the RUN/SS pin reaches 0.65V,

the LTC3736’s two controllers are enabled.

The start-up of V

OUT1

is controlled by the LTC3736’s

internal soft-start. During soft-start, the error amplifier

EAMP compares the feedback signal V

FB1

to the internal

soft-start ramp (instead of the 0.6V reference), which rises

linearly from 0V to 0.6V in about 1ms. This allows the

output voltage to rise smoothly from 0V to its final value,

while maintaining control of the inductor current.

The 1ms soft-start time can be increased by connecting

the optional external soft-start capacitor C

between the

RUN/SS and SGND pins. As the RUN/SS pin continues to

OPERATIO

rise linearly from approximately 0.65V to 1.3V (being

charged by the internal 0.7µA current source), the EAMP

regulates the V

FB1

proportionally linearly from 0V to 0.6V.

The start-up of V

OUT2

is controlled by the voltage on the

TRACK pin. When the voltage on the TRACK pin is less

than the 0.6V internal reference, the LTC3736 regulates

the V

FB2

voltage to the TRACK pin instead of the 0.6V

reference. Typically, a resistor divider on V

OUT1

is con-

nected to the TRACK pin to allow the start-up of V

OUT2

“track” that of V

OUT1

. For one-to-one tracking during start-

up, the resistor divider would have the same values as the

divider on V

OUT2

that is connected to V

FB2

Light Load Operation (Burst Mode or Continuous

Conduction) (SYNC/FCB Pin)

The LTC3736 can be enabled to enter high efficiency Burst

Mode operation or forced continuous conduction mode at

low load currents. To select Burst Mode operation, tie the

SYNC/FCB pin to a DC voltage above 0.6V (e.g., V

). To

select forced continuous operation, tie the SYNC/FCB to a

DC voltage below 0.6V (e.g., SGND). This 0.6V threshold

between Burst Mode operation and forced continuous mode

can be used in secondary winding regulation as described

in the Auxiliary Winding Control Using SYNC/FCB Pin dis-

cussion in the Applications Information section.

When a controller is in Burst Mode operation, the peak

current in the inductor is set to approximate 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 lower than the load current, the EAMP will

decrease the voltage on the I

pin. When the I

voltage

drops below 0.85V, the internal SLEEP signal goes high

and both external MOSFETs are turned off.

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

reducing the quiescent current that the LTC3736 draws.

The load current is supplied by the output capacitor. As

the output voltage decreases, the EAMP increases the I

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.

(Refer to Functional Diagram)

LTC3736

3736fa

When a controller is enabled for Burst Mode 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 reversing and 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

voltage. In this mode, the efficiency at light loads is lower

than in Burst Mode operation. However, continuous mode

has the advantages of lower output ripple and less inter-

ference with audio circuitry.

When the SYNC/FCB pin is clocked by an external clock

source to use the phase-locked loop (see Frequency

Selection and Phase-Locked Loop), the LTC3736 operates

in PWM pulse skipping mode at light loads. In this mode,

the current comparator I

CMP

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, though (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. However, it provides low current efficiency

higher than forced continuous mode, but not nearly as

high as Burst Mode operation. During start-up or a short-

circuit condition (V

FB1

or V

FB2

≤ 0.54V), the LTC3736

operates in pulse skipping mode (no current reversal

allowed), regardless of the state of the SYNC/FCB pin.

Short-Circuit Protection

When an output is shorted to ground (V

< 0.12V), the

switching frequency of that controller is reduced to 1/5 of

the normal operating frequency. The other controller is

unaffected and maintains normal operation.

The short-circuit threshold on V

FB2

is based on the smaller

of 0.12V and a fraction of the voltage on the TRACK pin.

This also allows V

OUT2

to start up and track V

OUT1

easily. Note that if V

OUT1

is truly short-circuited

OPERATIO

(Refer to Functional Diagram)

OUT1

= V

FB1

= 0V), then the LTC3736 will try to regulate

OUT2

to 0V if a resistor divider on V

OUT1

is connected to

the TRACK pin.

Output Overvoltage Protection

As a further protection, the overvoltage comparator (OV)

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-chan-

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

turned on until the overvoltage is cleared.

Frequency Selection and Phase-Locked Loop

(PLLLPF and SYNC/FCB Pins)

The selection of switching frequency is a tradeoff between

efficiency and component size. Low frequency operation

increases efficiency by reducing MOSFET switching losses,

but requires larger inductance and/or capacitance to main-

tain low output ripple voltage.

The switching frequency of the LTC3736’s controllers can

be selected using the PLLLPF pin.

If the SYNC/FCB is not being driven by an external clock

source, the PLLLPF can be floated, tied to V

or tied to

SGND to select 550kHz, 750kHz or 300kHz respectively.

A phase-locked loop (PLL) is available on the LTC3736 to

synchronize the internal oscillator to an external clock

source that connected to the SYNC/FCB pin. In this case,

a series RC should be connected between the PLLLPF pin

and SGND to serve as the PLL’s loop filter. The LTC3736

phase detector adjusts the voltage on the PLLLPF pin to

align the turn-on of controller 1’s external P-channel

MOSFET to the rising edge of the synchronizing signal.

Thus, the turn-on of controller 2’s external P-channel

MOSFET is 180 degrees out of phase with the rising edge

of the external clock source.

The typical capture range of the LTC3736’s phase-locked

loop is from approximately 200kHz to 1MHz, and is

guaranteed over temperature to be between 250kHz and

850kHz. In other words, the LTC3736’s PLL is guaranteed

to lock to an external clock source whose frequency is

between 250kHz and 850kHz.

LTC3736

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OPERATIO

(Refer to Functional Diagram)

Dropout Operation

When the input supply voltage (V

) decreases towards

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 external MOSFETs below safe

input voltage levels, an undervoltage lockout is incorporated

in the LTC3736. When the input supply voltage (V

) drops

below 2.3V, the external P- and N-channel MOSFETs and

all internal circuitry are turned off except for the undervolt-

age block, which draws only a few microamperes.

Peak Current Sense Voltage Selection and Slope

Compensation (IPRG1 and IPRG2 Pins)

When a controller is operating below 20% duty cycle, the

peak current sense voltage (between the SENSE

and SW

pins) allowed across the external P-channel MOSFET is

determined by:

∆ =

()

AV V

SENSE MAX

ITH

()

–.07

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

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

selects A = 5/3; tying IPRG to SGND 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. The peak sense

voltages for the two controllers can be independently

selected by the IPRG1 and IPRG2 pins.

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

slope compensation begins and effectively reduces the

peak sense voltage by a scale factor 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

∆

()

DUTY CYCLE (%)

SF = I/I

MAX

(%)

110

100

3736 F01

200

100

Figure 1. Maximum Peak Current vs Duty Cycle

Power Good (PGOOD) Pin

A window comparator monitors both feedback voltages

and the open-drain PGOOD output pin is pulled low when

either or both feedback voltages are not within ±10% of

the 0.6V reference voltage. PGOOD is low when the

LTC3736 is shut down or in undervoltage lockout.

2-Phase Operation

Why the need for 2-phase operation? Until recently, con-

stant frequency dual switching regulators operated both

controllers in phase (i.e., single phase operation). This

means that both topside MOSFETs (P-channel) are turned

on at the same time, causing current pulses of up to twice

the amplitude of those from a single regulator to be drawn

from the input capacitor. These large amplitude pulses

increase the total RMS current flowing in the input capaci-

tor, requiring the use of larger and more expensive input

capacitors, and increase both EMI and power losses in the

input capacitor and input power supply.

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LTC3736EGN#PBF

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Switching Voltage Regulators 2-Phase Synch Controller w/ Tracking

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