MIC2159 datasheet(7/17 Pages) MICREL

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Micrel

MIC2159

October 2006

M9999-101206

3. Soft-start.

For better understanding of the soft-start feature,

assume VIN = 12V and the MIC2159 is allowed to

power-up by un-grounding the COMP/EN pin. The

COMP pin has an internal 8.5µA current source that

charges the external compensation capacitor. As soon

as this voltage rises to 180mV (t = Cap_COMP ×

0.18V/8.5µA), the MIC2159 allows the internal VDD

linear regulator to power up and as soon as it crosses

the under-voltage lockout of 2.6V, the chip’s internal

oscillator starts switching. At this point in time, the

COMP pin current source increases to 40µA and an

internal 12-bit counter starts counting, which takes

approximately 2ms to complete. During counting, the

COMP voltage is clamped at 0.65V. After this counting

cycle the COMP current source is reduced to 8.5µA and

the COMP pin voltage rises from 0.65V to 0.95V, the

bottom edge of the saw-tooth oscillator. This is the

beginning of 0% duty cycle and it increases slowly

causing the output voltage to rise slowly. The MIC2159

has two hysteretic comparators that are enabled when

VOUT is outside ±3% of steady state. When the output

voltage reaches 97% of programmed output voltage then

the gm error amplifier is enabled along with the

hysteretic comparator. From this point onwards, the

voltage control loop (gm error amplifier) is fully in control

and will regulate the output voltage. Soft-start time can

be calculated approximately by adding the following four

time frames:

t1 = Cap_COMP × 0.18V/8.5µA

t2 = internal counter, approx 2ms

t3 = Cap_COMP × 0.3V/8.5µA

t4 =

8.5µ.

Cap_COMP

0.5

OUT

⋅

Soft-Start Time(Cap_COMP=100nF) = t1 + t2 +

t3 + t4 = 2.1ms + 2ms + 3.5ms + 1.8ms =

10ms

Current Limit

The MIC2159 uses the RDS(ON) of the top power

MOSFET to measure output current. Since it uses the

drain to source resistance of the power MOSFET, it is

not very accurate. This MOSFET scheme is adequate to

protect the power supply and external components

during a fault condition by cutting back the time the top

MOSFET is on if the feedback voltage is greater than

0.67V. In case of a hard short when feedback voltage is

less than 0.67V, the MIC2159 discharges the COMP

capacitor to 0.65V, resets the digital counter and

automatically shuts off the top gate drive, and the gm

error amplifier and the +/- 3% hysteretic comparators are

completely disabled and the soft-start cycles restarts.

This mode of operation is called the “hiccup mode” and

its purpose is to protect the down stream load in case of

a hard short. The circuit in Figure 1 illustrates the

MIC2159 current limiting circuit.

Figure 1. MIC2159 Current Limiting Circuit

The current limiting resistor RCS is calculated by the

following equation:

200µ

DS(ON)

⋅

Equation (1)

RIPPLE

LOAD

⋅

Where:

SWITCHING

OUT

RIPPLE

⋅

−

⋅

FSWITCHING = 400kHz

200µA is the internal sink current to program the

MIC2159 current limit.

The MOSFET RDS(ON) varies 30% to 40% with

temperature; therefore, it is recommended to add a 50%

margin to the load current (ILOAD) in the above equation

to avoid false current limiting due to increased MOSFET

junction temperature rise. It is also recommended to

connect RCS resistor directly to the drain of the top

MOSFET Q1, and the RSW resistor to the source of Q1

to accurately sense the MOSFETs RDS(ON). To make the

MIC2159 insensitive to board layout and noise

generated by the switch node, a 1.4Ω resistor and a

1000pF capacitor is recommended between the switch

node and GND. A 0.1µF capacitor in parallel with RCS

should be connected to filter some of the switching

noise.

Internal VDD Supply

The MIC2159 controller internally generates VDD for self

biasing and to provide power to the gate drives. This

VDD supply is generated through a low-dropout

regulator and generates 5V from VIN supply greater than

5V. For supply voltage less than 5V, the VDD linear

regulator is approximately 200mV in dropout. Therefore,

it is recommended to short the VDD supply to the input

supply through a 5Ω resistor for input supplies between

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