MIC2267 Datasheet, PDF (13/19 Pages) Micrel Semiconductor – Input Current Limiting Synchronous Buck Regulator

Micrel Inc.
LIMIT
Resistor values of between 750kΩ and 46kΩ should be
used to set the current limit between 0.1A and 1.6A. To
set the nominal LIMIT resistor value:
R_LIMIT = 75kΩ/I_LIMIT
In USB applications for example, two LIMIT resistors can
be used to switch between 1 unit load (100mA) and 5
unit loads (500mA).
Due to a minimum on time implemented during current
limit operation, there is lower limit to V_OUT where input
current limit is regulated. When V_OUT sees a near short
circuit at higher switching frequency, I_IN will be higher
than the set I_LIMIT. Circuit losses will tend to keep this to a
maximum of 250mA.
The limit can be found by:
V_OUTMIN = ((V_IN + 0.8) ⋅ F_SW ⋅ 200ns) − 0.8
e.g., for V_IN = 5v and F_SW = 1MHz, V_OUTMIN = 0.36V
COMP
As the MIC2267 uses a current mode control, the control
loop only requires a single pole/zero compensator to
optimise stability and transient response. The
recommended values are 11kΩ and 15nF. An additional
680pF capacitor can also be added to reject switching
frequency noise.
FB
Connect a resistor divider between the V_OUT load
terminal connection and output ground reference
connection to minimize resistive voltage drops affecting
load regulation.
For most applications, R2 can be set to 10kΩ and R2
can be found by:
R1 = R2(V_OUT/V_REF – 1)
SLOPEC
Connect a resistor between this pin and AGND as close
to the MIC2267 as possible to reduce the possibility of
noise adding into the control loop.
As described in the theory of operation, the ideal slope
can be calculated as:
R_SLOPEC = 2. F_SW . L . 67700/V_OUT
This value can be scaled towards ½ this value to ensure
stability up to and including 100% duty cycle.
MIC2267
FREQ
Connect a resistor between this pin and AGND as close
to the MIC2267 as possible to reduce the possibility of
switching noise causing frequency jitter.
Set the nominal switching frequency using:
F_SW = 1/ (C.R_FREQ)
Where nominal C = 10pF
INDUCTOR
The MIC2267 was designed to work with 3.3µH to 10µH
inductor values.
If a low ripple voltage output is a key design goal, then
larger value inductors will reduce switching ripple current
and output ripple voltage, but can also have larger DCR
values in small packages; which can reduce efficiency.
Inversely, if high efficiency is the key target, Lower value
inductors will increase switching ripple current and
therefore increase output ripple voltage, but will typically
have lower DCR values in small packages and can
improve efficiency.
As the MIC2267 uses input current limiting, care should
be taken that during a short circuit condition, the inductor
can operate with the power dissipated in it during this
fault condition. Helpfully, The MIC2267 switches the low
side driver off during a short circuit, which ensures most
of the power dissipation occurs within the IC. e.g.:
P_FAULT = 5v x 1A = 5W
V_NFET ≈ 0.7V
V_{L_DCR} = I_IN x DCR = 1A x 50mΩ = 0.05V
Power in the inductor DCR = 7% overall
dissipation.
P_{L_DCR} = 0.36W
The MIC2267 thermal limit protection will therefore limit
the power in the inductor and protect from over
dissipation. Protection is further improved by designing a
low thermal resistance connection between the inductor
and the IC. To achieve this, a short, wide PCB trace
from the inductor to the SW pin is recommended.
Connecting the inductor close to the SW pin is also good
design practice as it minimizes the area available for
radiating switching frequency harmonics around the local
area of the switching regulator.
January 2011
13
M9999-011711-A