MIC2829 datasheet(36/52 Pages) MICREL

36 / 52 page

Micrel Inc.

MIC2829

May 2010

M9999-051410-B

PD(MAX) (W)

TA (°C)

4.26

-40

3.75

-20

3.23

2.71

2.20

1.68

1.16

0.65

100

0.13

120

Table 6. Maximum Power Dissipation

It is good practice to not exceed the maximum power

dissipation of the device in order to avoid excessive

temperature rise or unexpected thermal shutdown.

HyperLight Load™ Mode

The HyperLight Load™ (HLL) buck regulators on the

MIC2829 use a proprietary control loop (patented by

Micrel). It has two modes of operation (HLL mode and

PWM mode).

The transition from HLL mode to PWM mode is

determined by the inductor ripple current. If the inductor

ripple current reaches below zero it is considered to be

in discontinuous mode (DCM). The HLL control loop will

control the switching in DCM using pulse frequency

modulation (PFM). As the load pulls the output voltage

below the monitored threshold, the HLL control loop

turns

the

topside

PMOS

transistor

for

predetermined time until the output voltage rises above

the monitored threshold. Once the upper threshold is

reached, the topside PMOS is switched off and the

voltage will then be slowly pulled down by the load. As

the load increases, the switching frequency increases.

By varying the switching frequency, the regulator only

switches when needed which improves efficiency by

reducing switching losses.

As the load increases and the inductor ripple current

rises above zero, the HLL regulator switches into

continuous conduction mode (CCM). The equation to

calculate the load when the HLL regulator goes into

continuous conduction mode may be approximated by

the following formula:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

)

OUT

LOAD

As shown in the equation, the load at which HLL

regulators transitions from HyperLight Load™ mode to

PWM mode is a function of the input voltage (VIN), the

output voltage (VOUT), the duty cycle (D), the inductance

(L) and the switching frequency (f). Note that the duty

cycle is approximately VOUT divided by VIN for buck

converters. The following graph shows the HLL regulator

switching frequency versus the output current. Since the

inductance range of MIC2829 is from 1µH to 2.2µH, the

device may then be tailored to enter HyperLight Load™

mode or PWM mode at a specific load current by

selecting the appropriate inductance. For example, in

Figure 4, when the inductance is 2.2µH the HLL

regulator will transition into PWM mode at a load of

approximately 30mA. Under the same condition, if 1µH

inductance is used, the MIC2829 will transition into PWM

mode at approximately 100mA.

DC4 Switching Frequency

vs. Output Current

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

100

1000

OUTPUT CURRENT (mA)

VIN = 5V

VOUT = 1.8V

COUT = 4.7µF

L = 2.2µH

L = 1µH

Figure 4. Switching Frequency with Various Inductance

In CCM, the HLL regulator works in pulse width

modulation (PWM) by controlling the PMOS transistor

off-time. To regulate the output voltage, the PMOS

transistor off-time is controlled. As the output voltage

decreases, the PMOS transistor off-time is decreased.

the

output

voltage

increases,

the

off-time

increased. This method of controlling the off-time

achieves the same goal as controlling the on-time as in

other PWM regulators by increasing or decreasing the

duty cycle of the PMOS transistor. In CCM, the

synchronous switching between the PMOS and the

NMOS is modulated at 4MHz for DC1, DC2 and DC4.

Due to the higher output voltage of DC3 (3V), the

switching frequency in CCM is at 2.5MHz. The HLL

regulators may reach the minimum-off-time limit at lower

input voltage and higher load currents. In order to

regulate at such high duty cycles, the HLL regulator

transitions into the on-time control scheme. During the

on-time control scheme, the off-time is set constant at

around (65ns), and the on-time is increased to deliver

more energy. By doing so, the duty cycle is increased,

and the output voltage maintains regulation even at

lower input voltages and extreme load situations. As a

result of increasing the on-time and fixing the off-time,

the switching frequency is lowered. In CCM, the

switching frequency is relatively constant, but at higher

output voltage and output current levels, the control may

transition into on-time control to regulate the output and

thus, lower the switching frequency.

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