EVAL5985 STMicroelectronics, EVAL5985 Datasheet - Page 10

• Manufacturer Part Number: EVAL5985
• Description: BOARD EVAL FOR L5985
• Manufacturer: STMicroelectronics
• Type: DC/DC Switching Converters, Regulators & Controllersr

Specifications of EVAL5985

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• Current - Output: 2A
• Voltage - Input: 3.3 ~ 18V
• Regulator Topology: Buck
• Frequency - Switching: 500kHz
• Board Type: Fully Populated
• Utilized Ic / Part: L5985
• Input Voltage: 2.9 V to 18 V
• Output Voltage: 0.6 V to 16 V
• Product: Power Management Modules
• Silicon Manufacturer: ST Micro
• Silicon Core Number: L5985
• Kit Application Type: Power Management - Voltage Regulator
• Application Sub Type: Step Down Switching Regulator
• Kit Contents: Board
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• For Use With/related Products: L5985
• Other names: 497-6387; EVAL5985

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 13006 Rev 5
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
L5985