Micrel, Inc.

MIC28511 Evaluation Board

April 9, 2015

2

Revision 1.0

Getting Started

1.

Connect VIN Supply

Connect a supply to the VIN and GND terminals,

paying careful attention to the polarity and the supply

meter and monitor input voltage at VIN and GND

terminals with a voltmeter. Do not apply power until

Step 4.

2.

Connect Load and Monitor Input

Connect a load to the VOUT and GND terminals. The

load can be either a passive or an active electronic

load. A current meter can be placed between the

VOUT terminal and load to monitor the output current.

Ensure the output voltage is monitored at the VOUT

terminal.

3.

Enable Input

The EN terminal has an on board 100k

Ω pull-up

resistor (R16) to VIN, which allows the output to be

turned on when PVDD exceeds its UVLO threshold.

An EN (J16) connector is provided on the evaluation

board for ease-of-access to the enable feature.

Applying an external logic signal on the EN terminal to

pull it low or using a jumper to short the EN terminal to

the GND terminal will disable the MIC28511 evaluation

board.

4.

Apply Power

to the set voltage.

Evaluation Board Description

The basic parameters of the evaluation board are:

• Input range: 4.6V to 60V

• Output range: 0.8V to 0.85V × V

(For more detailed information, refer to Typical

Characteristics section. Note that 0.85V is the maximum

duty cycle of the MIC28511 controller)

• 300kHz switching frequency

(Adjustable 200kHz to 680kHz)

Feedback Resistors

With Jumper J11 in place, the output voltage is set to 5.0V

as determined by the feedback dividers R1 and R11.

Jumper J8 sets the output voltage to 3.3V.

With jumper J7 in place the output is set by modifying R9,

as illustrated in Equation 1:











 +

×

=

9

R

1

R

1

V

V

REF

OUT

Eq. 1

Where:

R1 is 10.0kΩ.

With jumpers J11, J8, and J7 removed, the output

regulates at the 0.8V reference voltage. All other voltages

not listed can be set by modifying the R9 with Jumper J7

installed according to Equation 2:

REF

OUT

REF

V

V

V

R1

R9

-

×

=

Eq. 2

Jumper J12 shorts out the feedback and forces the

converter to operate open loop and approach 100% duty

cycle.

SW Node

switching waveform.

Current Limit

connected from ILIM to the SW node to decide the current

limit (see Figure 1).

Figure 1. MIC28511 Current Limiting Circuit

In each switching cycle of the MIC28511 converter, the

inductor current is sensed by monitoring the low-side

compared with the power ground (PGND) after a blanking

time of 150ns. In this way, the drop voltage over the

bottom FET generating the short current limit. The small

capacitor (C18) connected from ILIM pin to PGND filters

the switching node ringing during the off time which allows

a better short-limit measurement. The time constant

created by R22 and C18 should be much less than the

minimum off time.

value of the resistor (R22). If the absolute value of the

is lower than PGND and a short-circuit event is triggered.