MIC5156/5157/5158 Micrel, Inc.
MIC5156/5157/5158 8 August 2005
Charge Pump (MIC5157/5158 only)
The charge pump tripler creates a dc voltage across reservoir
capacitor C3. External capacitors C1 and C2 provide the nec
-
essary storage for the stages of the charge pump tripler.
The tripler’s approximate dc output voltage is:
V
CP
≈ 3 (V
DD
– 1)
where:
V
CP
= charge pump output voltage
V
DD
= supply voltage
The V
CP
clamp circuit limits the charge pump voltage to 16V
above V
DD
by gating the charge pump oscillator ON or OFF as
required. The charge pump oscillator operates at 160kHz.
The error amplifier uses the charge pump voltage to drive
the gate of the external MOSFET. It provides a constant load
of about 1mA to the charge pump. The error amplifier output
can swing to within 1V of V
CP
.
Although the MIC5157/8 is designed to provide gate drive
using its internal charge pump, an external gate drive sup-
ply voltage can be applied to V
CP
. When using an external
gate drive supply, V
CP
must not be forced more than 14V
higher than V
DD
.
When constant loads are driven, the ON/OFF switching of
the charge pump may be evident on the output waveform.
This is caused by the charge pump switching ON and rapidly
increasing the supply voltage to the error amplifier. The period
of this small charge pump excitation is determined by a number
of factors: the input voltage, the 1mA op-amp load, any dc
leakage associated with the MOSFET gate circuit, the size
of the charge pump capacitors, the size of the charge pump
reservoir capacitor, and the characteristics of the input voltage
and load. The period is lengthened by increasing the charge
pump reservoir capacitor (C3). The amplitude is reduced by
weakening the charge pump—this is accomplished by reduc-
ing the size of the pump capacitors (C1 and C2). If this small
burst is a problem in the application, use a 10µF reservoir
capacitor at C3 and 0.01µF pump capacitors at C1 and C2.
Note that the recovery time to repetitive load transients may
be affected with small pump capacitors.
Gate-to-Source Clamp
A gate-to-source protective voltage clamp of 16.6V protects
the MOSFET in the event that the output voltage is suddenly
forced to zero volts. This prevents damage to the external
MOSFET during shorted load conditions. Refer to “Charge
Pump” for normal clamp circuit operation.
The source connection required by the gate-to-source clamp
is not available on the adjustable version of the MIC5156.
Output Regulation
At start-up, the error amplifier feedback voltage (EA), or
internal feedback on fixed versions, is below nominal when
compared to the internal 1.235V bandgap reference. This
forces the error amplifier output high which turns on exter-
nal MOSFET Q1. Once the output reaches regulation, the
controller maintains constant output voltage under changing
input and load conditions by adjusting the error amplifier
output voltage (gate enhancement voltage) according to the
feedback voltage.
Out-of-Regulation Detection
When the output voltage is 8% or more below nominal, the
open-collector FLAG output (normally high) is forced low to
signal a fault condition. The FLAG output can be used to
signal or control external circuitry. The FLAG output can also
be used to shut down the regulator using the EN control.
Current Limiting
Super LDO Regulators perform constant-current limiting (not
foldback). To implement current limiting, a sense resistor
(R
S
) must be placed in the “power” path between V
DD
and
D (drain).
If the voltage drop across the sense resistor reaches 35mV,
the current limit comparator reduces the error amplifier out-
put. The error amplifier output is decreased only enough to
reduce the output current, keeping the voltage across the
sense resistor from exceeding 35mV.
Application Information
MOSFET Selection
Standard N-channel enhancement-mode MOSFETs are ac-
ceptable for most Super LDO regulator applications.
Logic-level N-channel enhancement-mode MOSFETs may
be necessary if the external gate drive voltage is too low
(MIC5156), or the input voltage is too low, to provide adequate
charge pump voltage (MIC5157/8) to enhance a standard
MOSFET.
Circuit Layout
For the best voltage regulation, place the source, ground,
and error amplifier connections as close as possible to the
load. See figures (1a) and (1b).
Figure 1a. Connections for Fixed Output
