MIC2169_09 Datasheet, PDF (7/15 Pages) Micrel Semiconductor – 500kHz PWM Synchronous Buck Control IC

MIC2169
Micrel
two hysteretic comparators that are enabled 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. From 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 × 0.18V/8.5μA
t2 = 12 bit counter, approx 2ms
t3 = Cap_COMP × 0.3V/8.5μA
t4
=
⎛
⎝⎜
V_OUT
V_IN
⎞
⎠⎟
×
0.5
×
Cap_COMP
8.5 μA
Soft-Start Time(Cap_COMP=100nF) = t1 + t2 + t3 +
t4 = 2.1ms + 2ms + 3.5ms + 1.8ms = 10ms
Current Limit
The MIC2169 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 not very accurate.
However, 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 MIC2169 discharges the COMP
capacitor to 0.65V, resets the digital counter and automatically
shuts off the top gate drive, and the g_m 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 a hard short. The circuit in Figure 1 illustrates
the MIC2169 current limiting circuit.
V_IN
C2
C_IN
0.1μF
HSD
2Ω
RCS
CS
LSD
Q1
MOSFET N
Q2
MOSFET N
L1 Inductor
1.4Ω
1000pF
V_OUT
C1
C_OUT
200μA
Figure 1. The MIC2169 Current Limiting Circuit
The current limiting resistor R_CS is calculated by the follow-
ing equation:
R_CS
=
R × _{DS(ON) Q1}
200μA
I_L
Equation (1)
I_L
= I _LOAD
+
2 (Inductor
1
Ripple
Current)
where:
Inductor Ripple Current =
( ) _VOUT
×
V_IN
V_IN – V_OUT
× F_SWITCHING
×
L
F_SWITCHING = 500kHz
200μA is the internal sink current to program the MIC2169
current limit.
The MOSFET R_DS(ON) varies 30% to 40% with temperature;
therefore, it is recommended that a 50% margin be added
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 the
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 MIC2169 insensitive to
board layout and noise generated by the switch node. For
this a 1.4Ω resistor and a 1000pF capacitor is recommended
between the switch node and ground. A 0.1μF capacitor, in
parallel with R_CS, should be connected to filter some of the
switching noise.
Internal V_DD Supply
The MIC2169 controller internally generates V_DD for self bias-
ing and to provide power to the gate drives. This V_DD supply
is generated through a low-dropout regulator and generates
5V from V_IN supply greater than 5V. For supply voltage less
than 5V, the V_DD linear regulator is approximately 200mV in
dropout. Therefore, it is recommended to short the V_DD supply
to the input supply through a 5Ω resistor for input supplies
between 2.9V to 5V.
MOSFET Gate Drive
The MIC2169 high-side drive circuit is designed to switch an
N-Channel MOSFET. The block diagram on page 6 shows a
bootstrap circuit, consisting of D1 and CBST. It supplies energy
to the high-side drive circuit. Capacitor CBST is charged while
the low-side MOSFET is on and the voltage on the VSW pin
is approximately 0V. When the high-side MOSFET driver is
turned on, energy from CBST is used to turn the MOSFET
on. As the MOSFET turns on, the voltage on the VSW pin
increases to approximately V_IN. Diode D1 is reversed biased
and CBST floats high while continuing to keep the high-side
MOSFET on. When the low-side switch is turned back on,
CBST is recharged through D1. The drive voltage is derived
from the internal 5V V_DD bias supply. The nominal low-side
gate drive voltage is 5V and the nominal high-side gate drive
voltage is approximately 4.5V due the voltage drop across D1.
An approximate 20ns delay between the high- and low-side
driver transition is used to prevent current from simultane-
ously flowing unimpeded through both MOSFETs.
MOSFET Selection
The MIC2169 controller works from input voltages of 3V to
13.2V and has an internal 5V regulator to provide power to
turn the external N-Channel power MOSFETs for high- and
low-side switches. For applications where V_IN < 5V, the internal
V_DD regulator operates in dropout mode, and it is necessary
that the power MOSFETs used are sub-logic level and are in
full conduction mode for V_GS of 2.5V. For applications when
V_IN > 5V; logic-level MOSFETs, whose operation is specified
March 2009
7
M9999-032409