ZL2008 datasheet(11/43 Pages) RENESAS

11 / 43 page

ZL2008

FN6859 Rev 4.00

Page 11 of 43

April 29, 2011

Power Conversion Overview

The ZL2008 operates as a voltage-mode, synchronous buck

converter with a selectable constant frequency pulse width

modulator (PWM) control scheme that uses external MOSFETs,

capacitors, and an inductor to perform power conversion.

Figure 5 illustrates the basic synchronous buck converter

topology showing the primary power train components. This

converter is also called a step-down converter, as the output

voltage must always be lower than the input voltage. In its most

simple configuration, the ZL2008 requires two external N-

channel power MOSFETs, one for the top control MOSFET (QH)

and one for the bottom synchronous MOSFET (QL). The amount

of time that QH is on as a fraction of the total switching period is

known as the duty cycle D, which is described by Equation 1:

During time D, QH is on and VIN – VOUT is applied across the

inductor. The current ramps up as shown in Figure 6.

When QH turns off (time 1-D), the current flowing in the inductor

must continue to flow from the ground up through QL, during

which the current ramps down. Since the output capacitor COUT

exhibits a low impedance at the switching frequency, the AC

component of the inductor current is filtered from the output

voltage so the load sees nearly a DC voltage.

Typically, buck converters specify a maximum duty cycle that

effectively limits the maximum output voltage that can be

realized for a given input voltage. This duty cycle limit ensures

that the lowside MOSFET is allowed to turn on for a minimum

amount of time during each switching cycle, which enables the

bootstrap capacitor (CB in Figure 5) to be charged up and provide

adequate gate drive voltage for the high-side MOSFET. See

section“High-side Driver Boost Circuit” on page 12 for more

details.

In general, the size of components L1 and COUT as well as the

overall efficiency of the circuit are inversely proportional to the

switching frequency, fSW. Therefore, the highest efficiency circuit

may be realized by switching the MOSFETs at the lowest possible

frequency; however, this will result in the largest component size.

Conversely, the smallest possible footprint may be realized by

switching at the fastest possible frequency but this gives a

somewhat lower efficiency. Each user should determine the

optimal combination of size and efficiency when determining the

switching frequency for each application.

The block diagram for the ZL2008 is illustrated in Figure 4. In

this circuit, the target output voltage is regulated by connecting

the differential VSEN pins directly to the output regulation point.

The VSEN signal is then compared to a reference voltage that has

been set to the desired output voltage level by the user. The error

signal derived from this comparison is converted to a digital

value with a low-resolution, analog to digital (A/D) converter. The

digital signal is applied to an adjustable digital compensation

filter, and the compensated signal is used to derive the

appropriate PWM duty cycle for driving the external MOSFETs in a

way that produces the desired output.

The ZL2008 has several features to improve the power

conversion efficiency. A non-linear response (NLR) loop improves

the response time and reduces the output deviation as a result of

a load transient. The ZL2008 monitors the power converter’s

operating conditions and continuously adjusts the turn-on and

turn-off timing of the high-side and low-side MOSFETs to optimize

the overall efficiency of the power supply. Adaptive performance

optimization algorithms such as dead-time control, diode

emulation, and frequency control are available to provide greater

efficiency improvement.

Power Management Overview

The ZL2008 incorporates a wide range of configurable power

management features that are simple to implement with no

external components. Additionally, the ZL2008 includes circuit

protection features that continuously safeguard the device and

load from damage due to unexpected system faults. The ZL2008

can continuously monitor input voltage, output voltage/current,

internal temperature, and the temperature of an external

thermal diode. A Power Good output signal is also included to

enable power-on reset functionality for an external processor.

All power management functions can be configured using either

pin configuration techniques (see Figure 7) or via the I2C/SMBus

interface. Monitoring parameters can also be pre-configured to

provide alerts for specific conditions. See Application Note

AN2033 for more details on SMBus monitoring.

VIN

VOUT

BST

COUT

CIN

FIGURE 5. Synchronous Buck Converter

OUT



(EQ. 1)

Time

VIN -VOUT

-VOUT

1 - D

ILPK

ILV

FIGURE 6. Inductor Waveform

ZL2008 Datasheet(PDF) 11 Page - Renesas Technology Corp

ZL2008 Datasheet(HTML) 11 Page - Renesas Technology Corp

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

Datasheet Download

Link URL