MIC2169B Datasheet, PDF (9/24 Pages) Micrel Semiconductor – 500kHz PWM Synchronous Buck Control IC

Micrel, Inc.
MOSFET, causing the output voltage to go up and
bringing V_OUT 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 V_IN = 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 V_IN crosses the 2.6V UVLO threshold,
the MIC2169B allows the internal V_DD 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 V_OUT is
within -3% of steady state. When the output voltage
reaches 97% of programmed output voltage then the g_m
error amplifier is enabled along with the hysteretic
comparator output is asserted high. This point onwards,
the voltage control loop (g_m 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
t4
=
⎜⎜⎝⎛
V_OUT
V_IN
⎟⎟⎠⎞
×
0.5 ×
Cap _ COMP
8.5µA
Soft-Start Time(Cap_COMP=100nF) = t1 + t2
+ t3 + t4 = 2.9ms + 2ms + 3.5ms + 1.6ms =
10ms
Current Limit
The MIC2169B uses the R_DS(ON) of the top power
MOSFET to measure output current. Since it uses the
drain to source resistance of the power MOSFET, it is
MIC2169B
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 g_m 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.
V_IN
C2
C_IN
0.1µF
VSW
RCS
CS
HSD
LSD
Q1
MOSFET N
Q2
MOSFET N
L1 Inductor
1000pF
V_OUT
C1
C_OUT
200 A
Figure 1. The MIC2169B Current Limiting Circuit
The current limiting resistor R_CS is calculated by the
following equation:
R_CS
=
R_DS(ON)Q1 × I_L
200µA
where:
I_L
= I_LOAD
+
Inductor
Ripple Current
2
Inductor Ripple Current =
V_OUT
×
(V_IN
V_IN
−
×
V_OUT
F_S × L
)
F_S = 500kHz
200µA is the internal sink current to program the
MIC2169B current limit.
The MOSFET R_DS(ON) varies 30% to 40% with
temperature; therefore, it is recommended to add a 50%
margin to the load current (I_LOAD) in the above equation
to avoid false current limiting due to increased MOSFET
junction temperature rise. It is also recommended to
connect R_CS resistor directly to the drain of the top
MOSFET Q1, and the R_SW resistor to the source of Q1 to
accurately sense the MOSFETs R_DS(ON). To make the
MIC2169B insensitive to board layout and noise
generated by the switch node, a 1.4Ω resistor and a
June 2009
9
M9999- 060309-A
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