LM3370 datasheet(13/23 Pages) NSC

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Operation Description

DEVICE INFORMATION

The LM3370, a dual high efficiency step down DC-DC con-

verter, delivers regulated voltages from input rails between

2.7V to 5.5V. Using voltage mode architecture with synchro-

nous rectification, the LM3370 has the ability to deliver up to

600mA per channel. The performance is optimized for sys-

tems where efficiency and space are critical.

There are three modes of operation depending on the cur-

rent required — PWM, PFM, and shutdown. PWM mode

handles loads of approximately 70mA or higher with 90%

efficiency or better. Lighter loads cause the device to auto-

matically switch into PFM mode to maintain high efficiency

with low supply current (I

Q = 20µA typ.) per channel.

The LM3370 can operate up to a 100% duty cycle (PFET

switch always on) for low drop out control of the output

voltage. In this way the output voltage will be controlled

down to the lowest possible input voltage.

Additional features include soft-start, under voltage lock out,

current overload protection, and thermal overload protection.

CIRCUIT OPERATION

During the first portion of each switching cycle, the control

block in the LM3370 turns on the internal PFET switch. This

allows current to flow from the input through the inductor to

the output filter capacitor and load. The inductor limits the

current to a ramp with a slope of

by storing energy in a magnetic field. During the second

portion of each cycle, the controller turns the PFET switch

off, blocking current flow from the input, and then turns the

NFET synchronous rectifier on. The inductor draws current

from ground through the NFET to the output filter capacitor

and load, which ramps the inductor current down with a

slope of

The output filter stores charge when the inductor current is

high, and releases it when low, smoothing the voltage across

the load.

PWM OPERATION

During PWM operation the converter operates as a voltage-

mode controller with input voltage feed forward. This allows

the converter to achieve excellent load and line regulation.

The DC gain of the power stage is proportional to the input

voltage. To eliminate this dependence, feed forward in-

versely proportional to the input voltage is introduced.

INTERNAL SYNCHRONOUS RECTIFICATION

While in PWM mode, the LM3370 uses an internal NFET as

a synchronous rectifier to reduce rectifier forward voltage

drop and associated power loss. Synchronous rectification

provides a significant improvement in efficiency whenever

the output voltage is relatively low compared to the voltage

drop across an ordinary rectifier diode.

CURRENT LIMITING

A current limit feature allows the LM3370 to protect itself and

external components during overload conditions. PWM

mode implements cycle-by-cycle current limiting using an

internal comparator that trips at 1200mA (typ.). If the outputs

are shorted to ground the device enters a timed current limit

mode where the NFET is turned on for a longer duration until

the inductor current falls below a low threshold, ensuring

inductor has more time to decay, thereby preventing run-

away.

PFM OPERATION

At very light loads, the converter enters PFM mode and

operates with reduced switching frequency and supply cur-

rent to maintain high efficiency.

The part will automatically transition into PFM mode when

either of two conditions are true, for a duration of 32 or more

clock cycles:

The NFET current reaches zero.

The peak PFET switch current drops below the I

MODE

level .

Supply current during this PFM mode is less than 20µA per

channel, which allows the part to achieve high efficiency

under extremely light load conditions. When the output drops

below the ‘low’ PFM threshold, the cycle repeats to restore

the output voltage to

∼1.2% above the nominal PWM output

voltage.

If the load current should increase during PFM mode (see

Figure 5) causing the output voltage to fall below the ‘low2’

PFM threshold, the part will automatically transition into

fixed-frequency PWM mode.

During PFM operation, the converter positions the output

voltage slightly higher than the nominal output voltage during

PWM operation, allowing additional headroom for voltage

drop during a load transient from light to heavy load. The

PFM comparators sense the output voltage via the feedback

pin and control the switching of the output FETs such that the

output voltage ramps between 0.8% and 1.6% (typical)

above the nominal PWM output voltage. If the output voltage

is below the ‘high’ PFM comparator threshold, the PFET

power switch is turned on. It remains on until the output

voltage exceeds the ‘high’ PFM threshold or the peak current

exceeds the I

PFM level set for PFM mode. The typical peak

current in PFM mode is:

PFM = 115mA + VIN/57

Ω

Once the PFET power switch is turned off, the NFET power

switch is turned on until the inductor current ramps to zero.

When the NFET zero-current condition is detected, the

NFET power switch is turned off. If the output voltage is

below the ‘high’ PFM comparator threshold (see Figure 5),

the PFET switch is again turned on and the cycle is repeated

until the output reaches the desired level. Once the output

reaches the ‘high’ PFM threshold, the NFET switch is turned

on briefly to ramp the inductor current to zero and then both

output switches are turned off and the part enters an ex-

tremely low power mode.

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