MIC22205 Datasheet, PDF (17/29 Pages) Micrel Semiconductor – 2A, Integrated, Switch, High-Efficiency

Micrel, Inc.
300kHz to 800kHz Operation
The frequency range can be lowered by adding an
additional resistor (R_CF) in parallel with the CF capacitor.
This reduces the amount of current used to charge the
capacitor, which reduces the frequency. The following
equation can be used to for frequencies between
800kHz to 300kHz:
− R_CFxC_CFxln_{⎜⎜⎝⎛}1+
1.0V
400μ A x R_CF
⎟⎟⎠⎞
=
t
R_CF > 2.9KΩ
RC Pin (Soft-Start)
The RC pin provides a trimmed 1µA current source/sink
for accurate ramp-up (soft-start). This allows the
MIC22205 to be used in systems that require voltage
tracking or ratio-metric voltage tracking at startup.
There are two ways of using the RC pin:
1. Externally driven from a voltage source
2. Externally attached capacitor sets output ramp-
up/down rate
In the first case, driving RC with a voltage from 0V to
V_REF will program the output voltage between 0 and
100% of the nominal set voltage, as shown in Figure 3.
In the second case, the external capacitor sets the ramp-
up and ramp-down time of the output voltage. The time
is given by:
t _RAMP
=
0.7 × C_RC
1×10 ^{− 6}
where t_RAMP is the time from 0 to 100% nominal output
voltage.
During start-up, a light load condition (I_OUT < 1.25A) can
lead to negative inductor current. Under these
conditions, the maximum ramp-up time should not
exceed the critical ramp-up time period. This will keep
the regulator in continuous mode operation when V_FB
reaches 90% of reference voltage.
Beyond the critical ramp-up time, the regulator is in
discontinuous mode, which leads to prolonged N-
channel MOSFET conduction, which in turn causes
negative inductor current.
MIC22205
The maximum C_RC value is calculated as follows:
C_RC
<
2.86 • C_OUT • L • F_SW
⎜⎜⎝⎛¹
_
V_OUT
V_IN
⎟⎟⎠⎞
• 10 ^−6
Pre-Bias Start-Up
The MIC22205 is designed for safe start-up into a pre-
biased output. This prevents large negative inductor
currents and excessive output voltage oscillations. The
MIC22205 starts with the low-side MOSFET turned off,
preventing reverse inductor current flow. The
synchronous MOSFET stays off until the Power Good
(PG) goes high after the V_FB is above 90 percent of V_REF.
A pre-bias condition can occur if the input is turned off,
and then immediately turned back on before the output
capacitor is discharged to ground. It is also possible that
the output of the MIC22205 could be pulled up or pre-
biased through parasitic conduction paths from one
supply rail to another in multiple voltage (VOUT) level
ICs such as a FPGA.
Figure 3 shows a normal start-up waveform. A 1µA
current source charges the soft-start capacitor C_RC. The
C_RC capacitor forces the V_RC voltage to come up slowly
(V_RC trace), which provides a soft-start ramp. This ramp
is used to control the internal reference (V_REF). The error
amplifier forces the output voltage to follow the V_REF
ramp from zero to the final value.
Figure 3. EN Turn-On Time − Normal Start-Up
August 2011
17
M9999-082511-A