MIC22950 Datasheet, PDF (13/22 Pages) Micrel Semiconductor – 10A Integrated Switch Synchronous Buck Regulator with Frequency Programmable to 2MHz

Micrel, Inc.
Alternatively, the Figure 3 can be used as a visual guide:
Switching Frequency
vs. CF Capacitance
2250
2000
1750
1500
1250
1000
750
500
250
0
0
250
500
750
1000
CF CAPACITANCE (pF)
Figure 3. Switching Frequency vs. CF Capacitance
This pin should not be over-ridden using an external
clock because the trigger pulses generated when the CF
pin reaches 1V are utilized internally.
Sequencing and Tracking
The MIC22950 provides additional pins to provide
up/down sequencing and tracking capability for
connecting multiple voltage regulators together.
EN/DLY Pin
The EN/DLY pin contains a trimmed, 1µA current source
which can be used with a capacitor to implement a fixed
desired delay in some sequenced power systems. The
threshold level for power on is 1.24V with a hysteresis of
20mV.
DELAY Pin
The DELAY pin also has a 1µA trimmed current source and
a 1µA current sink which acts with an external capacitor to
delay the operation of the Power On Reset (POR/PG)
output. This can be used also in sequencing outputs in a
sequenced system, but with the addition of a conditional
delay between supplies; allowing a first up, last down power
sequence.
After EN pin is driven high, V_OUT will start to rise (rate
determined by RC pin capacitor). As the FB pin voltage
goes above 90% of its nominal set voltage, DELAY pin
begins to rise as the 1µA source charges the external
capacitor. When the threshold of 1.24V is crossed,
POR/PG is asserted high and DELAY continues to
charge to a voltage SVIN. When FB falls below 90% of
nominal, POR/PG is asserted low immediately.
MIC22950
However, if EN pin is driven low, POR/PG will fall
immediately to the low state and DELAY pin will begin to
fall as the external capacitor is discharged by the 1µA
current sink. When the threshold of (V_TP +1.24V)-1.24V
is crossed (V_TP is internal voltage clamp, V_TP ≅ 0.9V),
V_OUT will begin to fall at a rate determined by the RC pin
capacitor. As the voltage change in both cases is 1.24V,
both rising and falling delays are matched at:
T_POR
=
1.24 × C _DELAY
1 × 10 ^−6
RC Pin
The RC pin provides a trimmed 1µA current source/sink
similar to the DELAY Pin for accurate ramp up (soft
start) and ramp down control. This allows the MIC22950
to be used in systems requiring 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 pin with a voltage from 0V to
V_REF will program the output voltage between 0% and
100% of the nominal set voltage.
In the second case, the external capacitor sets the ramp
up and ramp down rate of the output voltage. The rate 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. RC pin cannot
be left floating. Use a minimum capacitor value of 120p
for larger.
Tracking and Sequencing Examples
There are four distinct variations which are easily
implemented using the MIC22950. The two Sequencing
variations are Delayed and windowed. The two tracking
variants are ratio Metric and Normal. The following
diagrams illustrate methods for connecting two
MIC22950’s to achieve these requirements.
February 2010
13
M9999-021910-B