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LTC4089/LTC4089-5

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High Voltage Step Down Regulator

The power delivered from HVIN to HVOUT is controlled

by a 750kHz constant frequency, current mode step down

regulator. An external P-channel MOSFET directs this

power to OUT and prevents reverse conduction from OUT

to HVOUT (and ultimately HVIN).

A 750kHz oscillator enables an RS ﬂip-ﬂop, turning on the

internal 1.95A power switch Q1. An ampliﬁer and compara-

tor monitor the current ﬂowing between the HVIN and SW

pins, turning the switch off when this current reaches a

level determined by the voltage at VC. An error ampliﬁer

servos the VC node to maintain approximately 300mV

between OUT and BAT (LTC4089). By keeping the voltage

across the battery charger low, efﬁciency is optimized

because power lost to the battery charger is minimized

and power available to the external load is maximized. If

the BAT pin voltage is less than approximately 3.3V, then

the error ampliﬁer will servo the VC node to provide a

constant HVOUT output voltage of about 3.6V. An active

clamp on the VC node provides current limit. The VC node

is also clamped to the voltage on the HVEN pin; soft-start

is implemented by a voltage ramp at the HVEN pin using

an external resistor and capacitor.

An internal regulator provides power to the control circuitry.

This regulator includes an undervoltage lockout to prevent

switching when HVIN is less than about 4.7V. The HVEN

pin is used to disable the high voltage regulator. HVIN

input current is reduced to less than 2µA and the external

P-channel MOSFET disconnects HVOUT from OUT when

the high voltage regulator is disabled.

The switch driver operates from either the high voltage

input or from the BOOST pin. An external capacitor and

diode are used to generate a voltage at the BOOST pin that

is higher than the input supply. This allows the driver to

fully saturate the internal bipolar NPN power switch for

efﬁcient operation.

When HVOUT is below 3.95V the operating frequency

is reduced. This frequency foldback helps to control the

regulator output current during start-up and overload.

Ideal Diode from BAT to OUT

The LTC4089/LTC4089-5 has an internal ideal diode as

well as a controller for an optional external ideal diode. If

a battery is the only power supply available, or if the load

current exceeds the programmed input current limit, then

the battery will automatically deliver power to the load via

an ideal diode circuit between the BAT and OUT pins. The

ideal diode circuit (along with the recommended 4.7µF

capacitor on the OUT pin) allows the LTC4089/LTC4089-5

to handle large transient loads and wall adapter or USB

VBUS connect/disconnect scenarios without the need for

large bulk capacitors. The ideal diode responds within

a few microseconds and prevents the OUT pin voltage

from dropping signiﬁcantly below the BAT pin voltage.

A comparison of the I-V curve of the ideal diode and a

Schottky diode can be seen in Figure 3.

If the input current increases beyond the programmed

input current limit additional current will be drawn from the

battery via the internal ideal diode. Furthermore, if power

to IN (USB VBUS) or HVIN (high voltage input) is removed,

then all of the application power will be provided by the

battery via the ideal diode. A 4.7µF capacitor at OUT is

sufﬁcient to keep a transition from input power to battery

power from causing signiﬁcant output voltage droop. The

ideal diode consists of a precision ampliﬁer that enables a

large P-channel MOSFET transistor whenever the voltage

at OUT is approximately 20mV (VFWD) below the voltage

at BAT. The resistance of the internal ideal diode is ap-

proximately 200m . If this is sufﬁcient for the application

OPERATIO

Figure 3. LTC4089/LTC4089-5 Versus

Schottky Diode Forward Voltage Drop

CONSTANT

I0N

CONSTANT

R0N

CONSTANT

V0N

VFWD

IMAX

FORWARD VOLTAGE (V)

4089 F03

IFWD

SLOPE: 1/RDIO(ON)

SLOPE: 1/RFWD

SCHOTTKY

DIODE

LTC4089

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