Datasheet
GND
OUT1
CMP1
CMPREF
CMP2
V
REF
V
DD
LM27212
U1
Q1
Q2
L1
R8
Q3
Q4
L2
R9
D1
C4
D2
C5
+
C6
+
C7
R5
R4
V
CC
5
V
IN
U2
LM27222
U3
LM27222
C3
C2
R6
R7
SYNC1
SYNC2
OUT2
C1
R1
R2
R3
V
CC
LEN
IN
GND
CB
HG
SW
LG
V
CC
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
V
OUT
V
IN
*
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
R
DS_ON
to minimize conduction losses. If the input voltage range were 4V to 6V, the MOSFET selection should
be changed. First, much lower voltage FETs can be used, and secondly, high-side FET R
DS_ON
becomes a larger
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
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