LM43602PWPR datasheet(13/51 Pages) TI2

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VIN

-VD1

tON

D = tON / TSW

VSW

tOFF

TSW

ûiL

IOUT

ILPK

LM43602

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SNVSA36C – APRIL 2014 – REVISED OCTOBER 2017

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7.3 Feature Description

7.3.1 Fixed Frequency Peak Current-Mode Controlled Step-Down Regulator

The following operating description of the LM43602 will refer to the Functional Block Diagram and to the

waveforms in Figure 25. The LM43602 is a step-down Buck regulator with both high-side (HS) switch and low-

side (LS) switch (synchronous rectifier) integrated. The LM43602 supplies a regulated output voltage by turning

on the HS and LS NMOS switches with controlled ON time. During the HS switch ON time, the SW pin voltage

VSW swings up to approximately VIN, and the inductor current iL increases with a linear slope (VIN - VOUT) / L.

When the HS switch is turned off by the control logic, the LS switch is turned on after a anti-shoot-through dead

time. Inductor current discharges through the LS switch with a slope of -VOUT / L. The control parameter of buck

converters are defined as Duty Cycle D = tON / TSW, where tON is the HS switch ON time and TSW is the switching

period. The regulator control loop maintains a constant output voltage by adjusting the duty cycle D. In an ideal

Buck converter, where losses are ignored, D is proportional to the output voltage and inversely proportional to

the input voltage: D = VOUT / VIN.

Figure 25. SW Node and Inductor Current Waveforms in Continuous Conduction Mode (CCM)

The LM43602 synchronous Buck converter employs peak current mode control topology. A voltage feedback

loop is used to get accurate DC voltage regulation by adjusting the peak current command based on voltage

offset. The peak inductor current is sensed from the HS switch and compared to the peak current to control the

ON time of the HS switch. The voltage feedback loop is internally compensated, which allows for fewer external

components, makes it easy to design, and provides stable operation with almost any combination of output

capacitors. The regulator operates with fixed switching frequency in Continuous Conduction Mode (CCM) and

Discontinuous Conduction Mode (DCM). At very light load, the LM43602 will operate in PFM to maintain high

efficiency and the switching frequency will decrease with reduced load current.

7.3.2 Light Load Operation

DCM operation is employed in the LM43602 when the inductor current valley reaches zero. The LM43602 will be

in DCM when load current is less than half of the peak-to-peak inductor current ripple in CCM. In DCM, the LS

switch is turned off when the inductor current reaches zero. Switching loss is reduced by turning off the LS FET

at zero current and the conduction loss is lowered by not allowing negative current conduction. Power conversion

efficiency is higher in DCM than CCM under the same conditions.

In DCM, the HS switch ON time will reduce with lower load current. When either the minimum HS switch ON time

(tON-MIN) or the minimum peak inductor current (IPEAK-MIN) is reached, the switching frequency will decrease to

maintain regulation. At this point, the LM43602 operates in PFM. In PFM, switching frequency is decreased by

the control loop when load current reduces to maintain output voltage regulation. Switching loss is further

reduced in PFM operation due to less frequent switching actions.

LM43602PWPR Datasheet(PDF) 13 Page - Texas Instruments

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