L5981 STMicroelectronics, L5981 Datasheet - Page 18

• Manufacturer Part Number: L5981
• Description: IC REG SW STEP DOWN 1A 8-VFQFPN
• Manufacturer: STMicroelectronics
• Type: Step-Down (Buck)r

Specifications of L5981

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 18 V
• Current - Output: 1A
• Frequency - Switching: 250kHz ~ 1MHz
• Voltage - Input: 2.9 ~ 18 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-VFQFN, 8-VFQFPN
• Power - Output: 1.5W
• Output Voltage: 3 V
• Output Current: 1 A
• Input Voltage: 2.9 V to 18 V
• Switching Frequency: 220 KHz to 275 KHz
• Operating Temperature Range: - 40 C to + 150 C
• Mounting Style: SMD/SMT
• Duty Cycle (max): 100 %
• Low Dropout Operation: 100 % duty cycle
• For Use With: 497-6385 - BOARD EVAL FOR L5981
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Application information
5.4.1
18/37
Equation 13
where:
Equation 14
Equation 15
As seen in
following paragraph the guidelines to select the type II and type III compensation network
are illustrated.
Type III compensation network
The methodology to stabilize the loop consists of placing two zeros to compensate the effect
of the LC double pole, so increasing phase margin; then to place one pole in the origin to
minimize the dc error on regulated output voltage; finally to place other poles far away the
zero dB frequency.
If the equivalent series resistance (ESR) of the output capacitor introduces a zero with a
frequency higher than the desired bandwidth (that is: 2π * ESR * COUT < 1 / BW), the type
III compensation network is needed. Multi Layer Ceramic capacitors (MLCC) have very low
ESR (<1 mΩ), with very high frequency zero, so type III network is adopted to compensate
the loop.
In
(f
Equation 16
Z1
Figure 10
, f
Z2
) and three poles (f
Chapter 4.3
the type III compensation network is shown. This network introduces two zeros
f
Q
LC
=
=
----------------------------------------------------------------------------------------- -
----------------------------------------------------------------------- -
2π
two different kind of network can compensate the loop. In the two
R
f
Z1
OUT
⋅
P0
L
=
L C
G
, f
+
----------------------------------------------- -
2π C
⋅
⋅
LC
L C
C
Doc ID 13004 Rev 6
P1
OUT
⋅
s ( )
OUT
⋅
, f
1
OUT
P2
3
=
⋅
⋅
⋅
). They expression are:
R
1
(
R
------------------------------------------------------------------------ -
1
OUT
1
⋅
1
+
(
+
R
+
----------------------------
2π Q f ⋅
-------------- -
R
OUT
ESR
R
⋅
OUT
E SR
3
1
⋅
)
s
,
+
+
,
------------------------- -
2π f
ESR
LC
f
⋅
Z2
f
s
+
zESR
zESR
)
⎛
⎝
,
=
------------------ -
2π f
----------------------------- -
2π R
s
=
⋅
⋅
LC
------------------------------------------- -
2π ESR C
1
R
4
⎞
⎠
OUT
⋅
2
⋅
C
4
1
=
⋅
V
--------------
I
OUT
OUT
OUT
L5981