PFM STEP-UP DC/DC CONVERTER WITH HIGH EFFICIENCY AND LOW NOISE

HT9261(A)

第 6 页共 13 页

Though at very low load condition, the quiescent current of chip do effect efficiency certainly.The four

main energy loss of Boost structure DC-DC converter in full load are the ESR of inductor,the voltage of

Schottky diode, on resistor of internal N-channel MOSFET and its driver. In order to improve the efficiency,

HT9261 integrates low on-resistor N-channel MOSFET and well design driver circuits.The switch energy

loss is limited at very low level.

SELECTION OF THE EXTERNAL COMPONENTS

Thus it can be seen, the inductor and schottky diode affect the conversion efficiency greatly.The

inductor and the capacitor also have great influence on the output voltage ripple of the converter.So it

is necessary to choose a suitable inductor, a capacitor and a right schottky diode, to obtain high

efficiency, low ripple and low noise.Before discussion，we define

Inductor Selection

Above all, we should define the minimum value of the inductor that can ensure the boost DC-DC to

operate in the continuous current-mode condition

f

R

D

D

L

L

2

)

1

(

min

2





The above expression is got under conditions of continuous current mode, neglect Schottky

diode’s

voltage, ESR of

both inductor and capacitor. The actual value is greater that it. If inductor’s value is less

than

Lmin，the efficiency of DC-DC converter will drop greatly, and the DC-DC circuit will not be

stable.Secondly, consider the ripple of the output voltage,

Lf

Vin

D

I





Δ

Lf

DVin

R

D

vin

ax

L

2

)

1

(

Im

2







If inductor value is too small, the current ripple through it will be great. Then the current through diode and

power switch will be great. Because the power switch on chip is not ideal switch, the energy of switch will

improve. The efficiency will fall.

Thirdly，in general, smaller inductor values supply more output current while larger values startup with

lower input voltage and acquire high efficiency.

Aninductor value of 3uH to 1mH works well in most applications. If DC-DC converter delivers large

output current (for example: output current is great than 50mA), large inductor value is recommended

in order to improve efficiency. If DC-DC must output very large current at low input supply voltage, small

inductor value is recommended.

resistor，then the energy can be calculated by following expression：

Vout

Vin

-

Vout

D 

2

D)

-

load(1

L

R

r

Δη

