MIC2185_05 Datasheet, PDF (10/15 Pages) Micrel Semiconductor – Low Voltage Synchronous Boost PWM Control IC

MIC2185
PWM Operation
Switch NodeVoltage
(Low Side FET Drain)
5V/div
High Side FET
Gate Drive
5V/div
Low Side FET
Gate Drive
5V/div
Inductor Current
1A offset; 0.5A/dvi
V_OUT RippleVoltage
200mV/div
PWM ModeWaveform
V_IN = 3.3V V_OUT = 5V I_OUT = 0.75A
TIME (1µs/div)
Figure 5 - PWM mode waveforms
Figure 5 shows typical waveforms for PWM mode of operation.
The gate drive signal turns on the external low side MOSFET,
Q1, allowing the inductor current to ramp up. When the low
side MOSFET turns off and the high side MOSFET, Q2, turns
on, current flowing in the inductor forces the MOSFET drain
voltage to rise until the is clamped at approximately the output
voltage. The MIC2185 uses current mode control to improve
output regulation and simplify compensation of the control
loop. Current mode control senses both the output voltage
(outer loop) and the inductor current (inner loop). It uses the
inductor current and output voltage to determine the duty
cycle (D) of the buck converter. Sampling the inductor current
effectively removes the inductor from the control loop, which
simplifies compensation. A simplified current mode control
diagram is shown in figure 6.
I_inductor
V_IN
I_inductor
Gate Driver
I_inductor
I_inductor
V_COMP
Voltage
Divider
V_REF
Gate Drive at OUTN
T_ON
T_PER
Figure 6. PWM Control Loop
A block diagram of the MIC2185 PWM current mode control
loop is shown in Figure 1. The inductor current is sensed by
measuring the voltage across a resistor, Rsense. The current
sense amplifier buffers and amplifies this signal. A ramp is
added to this signal to provide slope compensation, which is
Micrel, Inc.
required in current mode control to prevent unstable operation
at duty cycles greater than 50%.
A transconductance amplifier is used as an error amplifier,
which compares an attenuated output voltage with a refer-
ence voltage. The output of the error amplifier is compared
to the current sense waveform in the PWM block. When the
current signal rises above the error voltage, the comparator
turns off the low side drive. The error signal is brought out to
the COMP pin (pin 4) allowing the use of external components
to stabilize the voltage loop.
Current Sensing and Overcurrent Protection
The inductor current is sensed during the switch on time by
a current sense resistor located between the source of the
MOSFET, Q1 and ground (R_SENSE in Figure 1). Exceeding
the current limit threshold will immediately terminate the
gate drive of the N-channel MOSFET. This forces the Q1 to
operate at a reduced duty cycle, which reduces the output
voltage. In a boost converter, the overcurrent limit will
not protect the power supply or load during a severe
overcurrent condition or short circuit condition. If the
output is short-circuited to ground, current will flow from the
input, through the inductor and output diode,D1, to ground.
Only the impedance of the source and components limits
the current.
The minimum input voltage, maximum output power and
the minimum value of the current limit threshold determine
the value of the current sense resistor. The two switch, syn-
chronous operation of the MIC2185 forces the converter to
always operate in the continuous mode because current can
flow both ways through the high side P-channel MOSFET.
The equations below will help to determine the current sense
resistor value.
Maximum Peak Current
The peak inductor current is equal to the average inductor
current plus one half of the peak to peak inductor current.
The peak inductor current is:
I_IND(pk)=
I_IND(ave)
+
1
2
× I_IND(pp)
( ( )) _IIND(pk)=
V_O × I_O
V_IN × η
+
V_L
× V_O − V_IN × η
2 × V_O × fs × L
where:
I_O is the maximum output current
V_O is the output voltage
V_IN is the minimum input voltage
L is the value of the boost inductor
f_S is the switching frequency
η is the efficiency of the boost converter
V_L is the voltage across the inductor
V_L may be approximated as V_IN for higher input voltage.
However, the voltage drop across the inductor winding
resistance and low side MOSFET on-resistance must be
accounted for at the lower input voltages that the MIC2185
can operate at:
MIC2185
10
October 2005