MIC2169B datasheet(9/24 Pages) MICREL

9 / 24 page

Micrel, Inc.

MIC2169B

June 2009

M9999- 060309-A

(408) 944-0800

MOSFET, causing the output voltage to go up and

bringing VOUT back in regulation.

Soft-Start

The COMP/EN pin on the MIC2169B is used for the

following three functions:

1. Disables the part by grounding this pin

2. External compensation to stabilize the voltage

control loop

3. Soft-start

For better understanding of the soft-start feature,

assume VIN = 12V, and the MIC2169B 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 250mV (t = Cap_COMP ×

0.25V/8.5µA) and VIN crosses the 2.6V UVLO threshold,

the MIC2169B allows the internal VDD linear regulator to

power up, and the chip’s internal oscillator starts

switching. At this point in time, the COMP pin current

source increases to 40µA and an internal 11-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 MIC2169B has one hysteretic

comparator whose output is asserted high when VOUT is

within -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 output is asserted high. 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 × 0.25V/8.5µA

t2 = 12 bit counter, approx 2ms

t3 = Cap_COMP × 0.3V/8.5µA

µA

COMP

Cap

OUT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

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

+ t3 + t4 = 2.9ms + 2ms + 3.5ms + 1.6ms =

10ms

Current Limit

The MIC2169B 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 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 MIC2169B discharges the COMP capacitor to 0.65V,

resets the digital counter and automatically shuts off the

top gate drive, the gm error amplifier is completely

disabled, the –3% hysteretic comparators is asserted

low, and the soft-start cycles restart from t2 to t4. 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

MIC2169B current limiting circuit.

L1 Inductor

VIN

VOUT

HSD

LSD

RCS

VSW

200 A

CIN

COUT

MOSFET N

0.1µF

1000pF

Figure 1. The MIC2169B Current Limiting Circuit

The current limiting resistor RCS is calculated by the

following equation:

µA

200

)

(

where:

Current

Ripple

Inductor

LOAD

Inductor Ripple Current =

()

OUT

−

FS = 500kHz

200µA is the internal sink current to program the

MIC2169B 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

MIC2169B insensitive to board layout and noise

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

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