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LTC4090/LTC4090-5

4090fa

OPERATION

The oscillator reduces the switch regulator’s operating

frequency when the voltage at the HVOUT pin is low (be-

low 2.95V). This frequency foldback helps to control the

output current during start-up and overload.

The high voltage regulator contains a power good com-

parator which trips when the HVOUT pin is at 2.8V. The PG

output is an open-collector transistor that is off when the

output is in regulation, allowing an external resistor to pull

the PG pin high. Power good is valid when the switching

regulator is enabled and HVIN is above 3.6V.

Ideal Diode From BAT to OUT

The LTC4090/LTC4090-5 have 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 LTC4090/LTC4090-5

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 desired input current increases beyond the pro-

grammed 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 approximately 215m

Ω.

If this is sufﬁcient for the application then no external

components are necessary. However if more conductance

is needed, an external P-channel MOSFET can be added

from BAT to OUT. The GATE pin of the LTC4090/LTC4090-5

drives the gate of the external PFET for automatic ideal

diode control. The source of the external MOSFET should

be connected to OUT and the drain should be connected

to BAT. In order to help protect the external MOSFET in

overcurrent situations, it should be placed in close thermal

contact to the LTC4090/LTC4090-5.

Figure 2. Input and Battery Currents as a Function of Load Current

IIN

ILOAD(mA)

300

400

500

400

500

300

4090 F02a

200

100

200

ILOAD

IBAT

(CHARGING)

IBAT

(IDEAL DIODE)

IIN

ILOAD(mA)

100

4090 F02b

ILOAD

IBAT

(CHARGING)

IBAT

(IDEAL DIODE)

ILOAD (mA)

300

400

500

400

500

300

4090 F02c

200

100

200

IIN

ILOAD

IBAT = ICHG

IBAT = ICL = IOUT

IBAT

(CHARGING)

IBAT

(IDEAL DIODE)

(a) High Power Mode/Full Charge

RPROG = 100k and RCLPROG = 2k

(a) Low Power Mode/Full Charge

RPROG = 100k and RCLPROG = 2k

(a) High Power Mode with

ICL = 500mA and ICHG = 250mA

RPROG = 100k and RCLPROG = 2k

Burst Mode is a registered trademark of Linear Technology Corporation

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