MIC2185 Datasheet, PDF (10/15 Pages) Micrel Semiconductor – Low Voltage Synchronous Boost PWM Control IC

MIC2185
PWM Operation
Switch Node Voltage
(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/div
V_OUT Ripple Voltage
200mV/div
PWM Mode Waveform
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 opera-
tion. 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 com-
pensation 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
required in current mode control to prevent unstable opera-
tion 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 compo-
nents 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, synchro-
nous 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
×
2
V_O −
× V_O ×
V_IN × η
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 resis-
tance and low side MOSFET on-resistance must be ac-
counted for at the lower input voltages that the MIC2185 can
operate at.
MIC2185
10
May 2002