GND

OUT1

CMP1

CMPREF

CMP2

U1

Q1

Q2

L1

R8

Q3

Q4

L2

R9

D1

C4

D2

C5

+

C6

+

C7

R5

R4

U2

LM27222

U3

LM27222

C3

C2

R6

R7

SYNC1

SYNC2

OUT2

C1

R1

R2

R3

LEN

IN

GND

CB

HG

SW

LG

LEN

IN

GND

CB

HG

SW

LG

10:

0.1 PF

1.5:

1.5:

121:

121:

60.4:

60.4:

1 PF,

6.3V

1 PF,

6.3V

0.33 PF

**

0.6 PH

30A

1 m:

270 PF x 3

ESR = 9 m:

/cap

*

0.6 PH

30A

*

**

0.33 PF

10 PF x 6

1 m:

LM27222

SNVS306B – SEPTEMBER 2004 – REVISED MARCH 2013

www.ti.com

TYPICAL APPLICATION CIRCUIT DESCRIPITON

The Application Example on the following page shows the LM27222 being used with the LM27212, a 2-phase

hysteretic current mode controller. Although this circuit is capable of operating from 5V to 28V, the components

are optimized for an input voltage range of 9V to 28V. The high-side FET is selected for low gate charge to

reduce switching losses. For low duty cycles, the average current through the high-side FET is relatively small

and thus we trade off higher conduction losses for lower switching losses. The low-side FET is selected solely on

loss factor than the switching losses. Of course with a lower input voltage, the input capacitor voltage rating can

be reduced and the inductor value can be reduced as well. For a 4V to 6V application, the inductor can be

reduced to 200nH to 300nH. The switching frequency of the LM27212 is determined by the allowed ripple current

in the inductor. This circuit is set for approximately 300kHz. At lower input voltages, higher frequencies are

possible without suffering a significant efficiency loss. Although the LM27222 can support operating frequencies

up to 2MHz in many applications, the LM27212 should be limited to about 1MHz. The control architecture of the

LM27212 and the low propagation times of the LM27222 potentially gives this solution the fastest transient

response in the industry.

Application Example

* Q1, Q3: 2 x Si7390DP

** Q2, Q4: 2 x Si7356DP

8

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