MIC2159 Datasheet, PDF (7/17 Pages) Micrel Semiconductor – SYNCHRONOUS-itty™ Step-Down Converter IC

Micrel
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 = ^{VOUT ⋅ 0.5 ⋅ Cap_COMP}
V_IN ⋅ 8.5µ.
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
October 2006
MIC2159
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:
R _CS
=
R_DS(ON) ⋅ I_L
200µ A
Equation (1)
I_L
= I_LOAD
+
1
2
⋅
I_RIPPLE
Where:
I_RIPPLE
=
V_OUT
⋅
V_IN
V_IN − V_OUT
⋅ F_SWITCHING
⋅L
F_SWITCHING = 400kHz
200µA is the internal sink current to program the
MIC2159 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 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 R_DS(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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M9999-101206