MIC23156 datasheet(17/32 Pages) MICROCHIP

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 2017 Microchip Technology Inc.

DS20005919A-page 17

MIC23156

In HLL mode, the inductor is charged with a fixed tON

pulse on the High-Side Drive (HSD) switch. After this,

the LSD is switched on and current falls at a rate of

VOUT/L. The controller remains in HLL mode while the

inductor falling current is detected to cross at approxi-

mately 200 mA. When the LSD (or tOFF) time reaches

its minimum, and the inductor falling current is no

longer able to reach this 200 mA threshold, the part is

in CCM mode and switching at a virtually constant

frequency.

Table 5-1 optimizes the inductor to output capacitor

combination for maintaining a minimum phase margin

of 45°.

TABLE 5-1:

MAXIMUM COUT vs. INDUCTOR

5.4

Duty Cycle

The typical maximum duty cycle of the MIC23156 is

80%.

5.5

Thermal Shutdown

When the internal die temperature of MIC23156

reaches 160°C, the internal driver is disabled until the

die temperature falls below 140°C.

5.6

Efficiency Considerations

Efficiency is defined as the amount of useful output

power, divided by the amount of power supplied, as

shown in Equation 5-2:

EQUATION 5-2:

EFFICIENCY

CALCULATION

There are two types of losses in switching converters:

DC losses and switching losses. DC losses are simply

the power dissipation of I2R. Power is dissipated in the

high-side switch during the on cycle. Power loss is equal

to the high-side MOSFET RDSON, multiplied by the

switch current squared. During the off cycle, the low-side

N-channel MOSFET conducts, also dissipating power.

Device operating current also reduces efficiency. The

product of the quiescent (operating) current and the

supply voltage represents another DC loss. The current

required in driving the gates on and off at a constant

3 MHz frequency, and the switching transitions, make up

the switching losses.

FIGURE 5-2:

Efficiency Under Load.

Figure 5-2 shows an efficiency curve. From a 10 mA

load to 1.5A, efficiency losses are dominated by quies-

cent current losses, gate drive and transition losses. By

using the HyperLight Load mode, the MIC23156 is able

to maintain high efficiency at low-output currents.

Over 200 mA efficiency loss is dominated by MOSFET

RDSON and inductor losses. Higher input supply

voltages will increase the gate-to-source threshold on

the internal MOSFETs, thereby reducing the internal

RDSON. This improves efficiency by reducing DC

losses in the device. All but the inductor losses are

inherent to the device. In which case, inductor selection

becomes increasingly critical in efficiency calculations.

As the inductors are reduced in size, the DC Resis-

tance (DCR) can become quite significant. The DCR

losses can be calculated as shown in Equation 5-3:

EQUATION 5-3:

CALCULATING DCR

LOSSES

From that, the loss in efficiency due to inductor

resistance can be calculated as in Equation 5-4:

EQUATION 5-4:

LOSS IN EFFICIENCY DUE

TO INDUCTOR

RESISTANCE

Efficiency loss due to DCR is minimal at light loads and

gains significance as the load is increased. Inductor

selection becomes a trade-off between efficiency and

size in this case.

Inductor

Minimum

COUT

Recommended

COUT

Maximum

COUT

0.47 µH

2.2 µF

4.7 µF

25 µF

1.0 µH

2.2 µF

15 µF

2.2 µH

2.2 µF

6.8 µF









Efficiency % =

VOUT IOUT

VIN  IIN

 100

100

1000

10000

OUTPUT CURRENT (mA)

VIN = 3.6V

VIN = 5V

VIN = 2.7V

VIN = 4.2V

COUT = 2.2 µF

L = 1 µH

Efficiency (VOUT = 1.8V) vs.

Output Current

PDCR = IOUT

 DCR









Efficiency Loss = 1 –

VOUT IOUT

VOUT  IOUT  PDCR

[] 100

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