MIC2130 Datasheet, PDF (10/20 Pages) Micrel Semiconductor – High Voltage Synchronous Buck Control IC with Low EMI Option

Micrel, Inc.
At startup, the Soft-start MOSFET (SS_FET) is released
and C_SS starts to charge at the rate dV_SS/dt–2µA/C_SS.
The PNP transistor’s emitter (COMP) starts to track V_SS
(plus a junction voltage ≈0.65). When (COMP) reaches
the lower end of the PWM ramp voltage at 1.10V,
switching pulses will begin to drive the power MOSFETs.
This voltage rise continues on the COMP pin until the
control loop reaches the regulation point. During this soft
start period, the gate drive pulses to the MOSFET will
start at the minimum pulse width and increase up to the
duty cycle D required for regulation. The COMP voltage
can be anywhere from 1.1V to 2.1V which corresponds
to a duty cycle D of 0-85%. V_SS will however, continue to
rise as the PNP base-emitter junction becomes reverse
biased. The SS pin is allowed to rise to 2.5V (four diode
drops) max to allow fast response to fault conditions.
During large over current or short circuit conditions, i.e.,
where current limit is detected and V_OUT is <60% of
nominal, the SS_FET is momentarily switched on. This
discharges C_SS to ~150mV at which point, it re-starts the
soft start cycle once again. During soft start, hysteretic
comparators are disabled.
Duty Cycle D can be written in terms of V_COMP
D = (0.85) x V_COMP – (0.935) or V_COMP = (D + 0.935)/0.85
Example: V_IN = 12V; V_OUT = 3.3V; D = V_OUT/V_IN = 0.275
V_COMP = 1.424V; i.e. the steady-state DC Value of V_COMP
when D = 0.275
T1 is the time for V_COMP to charge up to 1.1V, therefore,
V_SS is one diode drop below V_COMP.
T2 is the time for V_COMP to charge up to (D + 0.935)/ 0.85
+ 1.1V.
Soft Start time = T1 + T2
Where
T1 = (1.1-Vdiode) x C_SS/2µA; the time until output
pulsing starts at minimum duty.
And,
T2 = (1/0.85) x D x C_SS/ 2µA; the time until output
pulsing increases to D.
The compensation capacitors at the COMP pin (C_COMP =
Cc1+Cc2 in Figure 4) will also need to charge up to
V_COMP. This charging time starts as soon as MOSFET
(SS_FET) is released. Depending upon the size of the
C_COMP, the charging time could be greater than T1+T2.
C_COMP could be used for the Soft Start cap by leaving SS
pin open.
Tc_COMP = (1/0.85) x D x C_COMP/5µA: The time until output
pulsing increases to D.
April 2008
MIC2130/1
Protection
There exits four different types of output protection.
1. Output “hard short” over current
2. Output “soft short” over current
3. Output under voltage
4. Output over voltage
Current Limit
The MIC2130/31 uses the RDS_ON of the low-side
MOSFET to sense overcurrent conditions. The lower
MOSFET is used because it displays much lower
parasitic oscillations during switching then the upper
MOSFET. Using the low-side MOSFET RDS_ON as a
current sense is an excellent method for circuit
protection. This method will avoid adding cost, board
space and power losses taken by discrete current sense
resistors.
Hard Short
Generally, the MIC2130/31 current limit circuit acts to
provide a fixed maximum output current until the
resistance of the load is so low that the voltage across it
is no longer within regulation limits. At this point (60% of
nominal output voltage), the part employs Hiccup mode.
During Hiccup mode, the output pulses stop and the soft
start cap is discharged and soft start mode begins. After
the soft start time, if the output voltage is still 60% low,
then the process repeats again and continues until the
short is removed. Hard short current mode is initiated to
protect down stream loads from excessive current and
also reduces overall power dissipation in the PWM
converter components during a fault.
Soft Short
Before “Hard Short” mode (also called “hiccup mode”)
occurs “soft short” current limiting is provided to prevent
system shutdown or disturbance if the overload is only
marginal. When the load current exceeds the current
limit by only a few ma for a short time (milliseconds) then
the hard short mode is not desired. Instead, the “Soft
Short” loop is used. When the current limit comparator
senses an over current it then starts to discharge the SS
Cap with a 40µA current source. The current limit
comparator gets reset every cycle so if the short still
exist during the next cycle then the SS cap will continue
to get discharged with the 40µA current source. The
comp pin follows the SS pin (Figure 4) and the gm
control loop will lower the output voltage accordingly for
as long as the short exists. So, instead of shutting down
the output as in a hard short, the output is gently and
slightly reduced until the over current condition
discontinues. If however the short increase to the point
of lowering the output to 60%, then hard short will result.
The fast hysteretic control loop (FHyCL) is initiated by a
6% drop in output voltage and it is not desired during an
over current condition therefore, the FHyCL feature will
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M9999-042108-C