L5987 STMicroelectronics, L5987 Datasheet - Page 11

• Manufacturer Part Number: L5987
• Description: IC REG STEPDOWN 3A 8VFQFPN
• Manufacturer: STMicroelectronics
• Type: Step-Down (Buck)r

Specifications of L5987

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.6 V ~ 18 V
• Current - Output: 3A
• Frequency - Switching: 250kHz
• Voltage - Input: 2.9 V ~ 18 V
• Operating Temperature: -40°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: 8-VFQFN, Exposed Pad
• Output Voltage: 0.6 V
• Output Current: 3 A
• Output Power: 1.5 W
• Input Voltage: 2.9 V to 18 V
• Switching Frequency: 250 KHz
• Mounting Style: SMD/SMT
• Duty Cycle (max): 100 %
• Low Dropout Operation: 100 % duty cycle
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

L5987
5.2
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
divider to regulate the output voltage (see
steps of 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 R1and R2 is the resistor
Doc ID 14972 Rev 3
SR
OUT
SS
=
TIME
SR
Figure
VREF
=
32 64
-----------------
Fsw
7). The soft-start stair case consists of 64
⋅
⋅
⎛
⎝
1
+
R1
------- -
R2
⎞
⎠
TIME
is 8 ms.
Functional description
REF
) of the error
11/41