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LTC3372

Rev. A

For more information www.analog.com

OPERATION

The LTC3372 is a single IC that combines a high voltage

(HV) buck (step-down) DC/DC switching regulator con-

troller and a total of four configurable low voltage (LV)

buck regulators.

Main Control Loop of HV Buck Controller

TheHVcontrollerusesaconstantfrequency,currentmode

buck (step-down) architecture. During normal operation,

the external top MOSFET is turned on when the clock sets

the RS latch, and is turned off when the main current

comparator, ICMP, resets the RS latch. The peak inductor

current at which ICMP trips and resets the latch is con-

trolled by the voltage on the ITH pin, which is the output

of the error amplifier, EA. The error amplifier compares

an internal 1.2V reference voltage to the feedback voltage

generated by an internal resistor divider connected to the

VOUT/EXTVCC pin. The divider can be selected to program

an output of either 3.3V to 5V. When the load current

increases, it causes a slight decrease in VFB relative to

the reference, which causes the EA to increase the ITH

voltage until the average inductor current matches the

new load current.

After the top MOSFET is turned off each cycle, the bottom

MOSFET is turned on until either the beginning of the next

clock cycle, or the inductor current starts to reverse, as

indicated by the reverse current comparator IREV (at light

load in pulse-skipping mode or Burst Mode).

INTVCC Power

Power for the top and bottom MOSFET drivers and some

otherinternalcircuitryisderivedfromtheINTVCCpin.When

the VOUT/EXTVCC pin is tied to a voltage less than 4.7V,

the VIN LDO (low dropout linear regulator) supplies 5.1V

from VIN to INTVCC. If VOUT/EXTVCC is taken above 4.7V,

the VIN LDO is turned off and an internal PMOS switch

connects VOUT/EXTVCCtoINTVCC.UsingtheVOUT/EXTVCC

pin allows the INTVCC power to be derived from the high

efficiency HV buck regulator output.

ThetopMOSFETdriverisbiasedfromthefloatingbootstrap

capacitor, CB, which normally recharges during each cycle

throughanexternaldiode,DB,whenthetopMOSFETturns

off. If the input voltage, VIN, decreases to a voltage close

to VOUT, the loop may enter dropout and attempt to turn

on the top MOSFET continuously. The dropout detector

detects this and forces the top MOSFET off for a short

time every tenth cycle to allow CB to recharge, resulting

in an effective duty cycle of 98%.

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

Ittypicallytakes1.2msfortheinternalbiascircuits(includ-

ing INTVCC) to be ready after the first time either the RUN

pin or any of the EN1-4 pins rises above 730mV. During

the 1ms internal bias circuit startup time, neither the HV

or LV regulators are enabled. The HV controller is enabled

when the RUN pin is pulled above 1.2V and the internal

bias is enabled. Pulling RUN pin below 1.16V disables

the HV controller.

The RUN pin has an internal 0.5μA current that pulls up

the pin to enable the HV controller. Alternatively, the RUN

pin may be externally pulled up or driven by logic. The

RUN pin can tolerate up to 65V (absolute maximum), so

it can be conveniently tied to VIN in an always-on applica-

tion in which the controller is enabled continuously and

never shut down.

The RUN pin can also be used as an undervoltage lockout

(UVLO) by connecting it to the midpoint of an external re-

sistordividernetworkofVIN(seeApplicationsInformation

section).

The start-up of the controller’s output voltage VOUT is

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

voltage on the TRACK/SS pin is less than the 1.2V internal

reference,theHVcontrollerregulatestheVFBvoltagetothe

TRACK/SS pin voltage instead of the 1.2V reference. This

allowstheTRACK/SSpintobeusedtoprogramasoft-start

by connecting an external capacitor from the TRACK/SS

pin to GND. An internal 10μA pull-up current charges this

capacitorcreatingavoltagerampontheTRACK/SSpin.As

the TRACK/SS voltage rises linearly from 0V to 1.2V and

beyond, the output voltage VOUT rises smoothly from zero

to its final value. Alternatively the TRACK/SS pin can be

used to cause the start-up of VOUT to track that of another

supply. Typically, this requires connecting to the TRACK/

SS pin an external resistor divider from the other supply

to ground (see the Applications Information section).

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