L5981 STMicroelectronics, L5981 Datasheet - Page 10

• 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

Functional description
4.2
10/37
Soft-start
The soft-start is essential to assure correct and safe start up of the step-down converter. It
avoids inrush current surge and makes the output voltage increases monothonically.
The soft-start is performed by a staircase ramp on the non-inverting input (V
amplifier. So the output voltage slew rate is:
Equation 1
where SR
to regulate the output voltage (see
9.5 mV each one, from 0 V to 0.6 V. The time base of one step is of 32 clock cycles. So the
soft-start time and then the output voltage slew rate depend on the switching frequency.
Figure 7.
Soft-start time results:
Equation 2
For example with a switching frequency of 250 kHz the SS
VREF
Soft-start scheme
is the slew rate of the non-inverting input while R1 and R2 the resistor divider
Doc ID 13004 Rev 6
SR
Figure
OUT
SS
=
TIME
7). The soft-start stair case consists of 64 steps of
SR
VREF
=
32 64
-----------------
Fsw
⋅
⋅
⎛
⎝
1
+
R1
------- -
R2
⎞
⎠
TIME
is 8 ms.
REF
) of the error
L5981